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Full text of "IS 3043: Code of Practice for Earthing (First Revision)"

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Disclosure to Promote the Right To Information 

Whereas the Parliament of India has set out to provide a practical regime of right to 
information for citizens to secure access to information under the control of public authorities, 
in order to promote transparency and accountability in the working of every public authority, 
and whereas the attached publication of the Bureau of Indian Standards is of particular interest 
to the public, particularly disadvantaged communities and those engaged in the pursuit of 
education and knowledge, the attached public safety standard is made available to promote the 
timely dissemination of this information in an accurate manner to the public. 




Mazdoor Kisan Shakti Sangathan 
'The Right to Information, The Right to Live'' 



Jawaharlal Nehru 
'Step Out From the Old to the New" 



IS 3043 (1987, Reaffirmed 2005) : Code of Practice for 
Earthing (First Revision). UDC 621.316.99 : 006.76 



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Satyanarayan Gangaram Pitroda 
Invent a New India Using Knowledge 



Bhartrhari — Nitisatakam 
"Knowledge is such a treasure which cannot be stolen" 





(Reaffirmed 2006) 



IS : 3043 - 1987 

(Reaffirmed 2006) 



Indian Standard 

CODE OF PRACTICE FOR EARTHING 

(First Revision) 



Fourth Reprint JUNE 2007 
(Including Amendment No. 1) 



UDC 621316.99 : 006.76 



© Copyright 1988 

BUREAU OF INDIAN STANDARDS 

MANAK BHAVAN, 9 BAHADUR SHAH ZAFAR MARG 
NEW DELHI 110002 

Price Rs 650.00 September 1988 



AMENDMENT NO. 1. JANUARY 2006 

TO 

IS 3043 : 1987 CODE OF PRACTICE FOR EARTHING 

(First Revision) 

{Page 19, clause 9.2.1) — Substitute the following for the existing 
formula: 



/l«"-T— V --j|-~ ohms 



.4-/ 

(ET 20) 



AMENDMENT NO. 2 JANUARY 2010 

TO 

IS 3043 : 1987 CODE OF PRACTICE FOR EARTHING 

(First Revision) 

{Page 17, clause 8.6) — Add the following new sub-clause after 8.6: 

8.6.1 Corrosion Allowance — On an average, steel corrodes about six times as fast as copper when placed in 
soil. The extent of corrosion depends upon the properties of soil The generally accepted correlation between the 
electrical resistivity of soil and its corrosivity is as indicated in the table below: 

Soil Resistivity and Corrosion 



Range of Soil Resistivity 
(ohm-metres) 


(ClassofSoil) 


Less than 25 

25-50 

50-100 
Above 100 


Severely corrosive 
Moderately corrosive 

Mildly corrosive 
Very mildly corrosive 



This following methods can be adopted to safeguard Conductor against excessive corrosion: 

a) Use of cathodic protection, and 

b) Use current conducting, corrosion resistant coating on steel (for example, zinc coating). 

The zinc coating on the tubes shall be in accordance with IS 4736 : 1986 'Hot dip zinc coatings on 
mild steel tubes ijirst revision) with coating thickness 150 microns, Min. 

a) Use steel conductor with large cross-section having allowance for corrosion. 

Based on the results of the field studies on rates of corrosion, the following allowances in cross- 
sectional area of the earthing conductor are recommended to take the effect of corrosion into 
account. 

Allowances in Cross-Sectional Area of the Earthing Conductor to Take the Effect of 

Corrosion into Account 



Type of Laying of the Earth Conductor 


Allowances to be 
Considered in Sizing 


a) 


Conductors laid to soils having resistivity 
greater then 100 ohni-meters 


(No allowance) 


b) 


Conductors laid in soils having resistivity 
from 25 to 1 00 ohm-meters 


1 5 percent 


c) 


Conductor laid in soils having resistivity 
lower than 25 ohm-meters or where treatment 
of soil around electrode is carried out 


30 percent 



For the purpose of determining the allowance to be made for corrosion, the minimum resistivity of 
the soil encountered at the location of grounding electrodes to be considered The. resistivity will be 
the minimum in wet weather. Thus, for very mildly corrosive soils, steel conductors meeting the 
stability and mechanical requirement are adequate. However, the steel conductors in the soil of other 
types should be at least 6 mm thick if it is steel flat and have a diameter of at least 16 mm if it is in 
the form of steel rod. 



Price Group 3 



Amend No. 1 to IS 3043 : 1987 

{Page 19, clause 9.2.1, para 1) — Substitute the following for the existing formula. 

(Page 20, clause 9.2.2, para 1) — Substitute the following for the existing fomnula: 

dfT I ft 

(Page 20, clause 9.2.2, /;ara 4) — Substitute the following for the existing 

'Pipes may be of cast iron of not less than 100 mm diameter, 2.5 to 3 m long and 13 mm thick. Such pipes 
cannot be driven satisfactorily and may, therefore, be more expensive to install than plates for the same effective 
area. Alternatively, mild steel water-pipes of 38 to 50 mm diameter are sometimes employed. These can be 
driven but are less durable than copper rods. Alternatively, 40 mm diameter Gl pipe in treated earth pit or 40 
mm diameter MS rod can be directly driven in virgin soil. The earth rod shall be placed at 1.250 m below 
ground.' 

(Page 21, clause 9.2.3, para 1) — Substitute the following for the existing formula alongwith its terms: 

2ftff 

where 

p = resistivity of soil (^.m) (assumed uniform): 

/ ~ length of the strip in cm; and 

t = width (strip) or twice the diameter (conductors) in cm 

(Page 24, Fig. 14) — Substitute the figure given on page 3 of this Amendment for the existing: 

(Page 25, Fig. 15) — Substitute the figures given on pages 4 and 5 of this Amendment for the existing: 

(Page 49, clause 2i}.6,2,2, first para, last sentence) — Substitute the following for the existing: 

'It is recommended that the duration of earth fault current should be taken as one second for 66 kV and above 
voltage level substations; and 3 seconds while designing earth grids for all other voltage levels below 66 kV.' 

[Page 49, c/c/t/5e 20.6*2.3(a), second sentence] — Substitute ' 1 and 3 seconds'/or '3 seconds' 

[Page 49, clause 20.6.2.3(a)] — Add the following new sentence at the end: 
'For corrosion allowance, see 8. 6. 1.' 



Amend No. 1 to IS 3043 : 1987 



M* 







Tf> i% " ^1 



NOTE — After laying the earth from the earth bus to the electrode through the PVC conduits at the pit entry conduits should be sealed with 
bitumen compound. 



All dimensions in millimetres. 
FIG. 14 TYPICAL ARRANGEMENT OF PIPE ELECTRODE 



CA5T ipCNOR 
CJ COVSR 



SZ2ZiSfeSSiS?S 




■zZ )> 1u G 3 3~RP 



/ OJT^lD'^ SUR-ACc S^-JOUIX 



I B^TUMirg, 



HOLTS &M<jTS. ':^it.Z< Nu^ 

A WASHED tA^Tfcft - XiNG, Tht 

OJrSIDt: SURFACE S-ttJLE 



ft 
s 






15A Earthing with G1 Plate 
All dimensions in millimetres. 

FIG. 15 TYPICAL ARRANGEMENT OF PLATE ELECTRODE — Conf/wecy 



I- 
u 



Amend No. 1 to IS 3043 : 1987 



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(ET 20) 



Reprography Unit, BIS New Delhi, India 



IS : 3043 - 1987 



Indian Standard 
CODE OF PRACTICE FOR EARTHING 
(First Revision) 



Electrical Installations Sectional Committee, ETDC 20 

Chairman 

SHRI M. L. DONGRE 

M-3 Satyam, 88 Sioii Circle, Bombay 400022 

Representing 
Engineer-in-Chief s Branch, Army Headquarters (Ministry of 



Members 
SHRi P. ANANTHARAMAN 



Defence), New Delhi 
Tata Consulting Engineers, Bombay 
Siemens India Ltd, Bombay 

Federation of Electricity Undertaking oflndia, Bombay 
Larsen & Toubro (Construction Group), Madras 
Railway Board (Ministry of Railways), New Delhi 



SHRi S. K. SHANGARl (Alternafe) 
SHRi P. D. BAGADE 

SHRI R. K. KAUL {Alternate) 
SHRI V. S. BHATIA 

SHRi M. M. SHETHNA {Alternate) 
SHRI K. V, CHAUBAL 

SHRI K . S . J O S H I {A Item ate) 
SHRI R. R. CHOUDHURl 

SHRI N. BALASUBRAMANIAN {Alternate) 
CHIEF ELECTRICAL ENGINEER 

DEPUTY DIRECTOR STANDARDS (ELEC)-Di, 
RDSO {Ai tern ale) 
CHIEF ELECTRICAL INSPECTOR TO GOVERNMENT Chief Electrical Inspector to Government ofTamil Nadu, Madras 

OF TAMIL NADU 

ELECTRICAL INSPECTOR (TECHNICAL) TO 
GOVERNMENT or TAMIL NADU {Alternate) 
CHIEF ENGINEER (ELEC)-l Central Public Works Department, New Delhi 

SUPERINTENDENT SURVEYOR or WORKS 
(ELEC)-l {Alternate) 
SHRI DEVENDER NATH Larsen &ToubroLtd, Bombay 

SHRI T. P. R. SARMA {Alternate) 
SHRI K. W. DHARMADHIKARI 

DR V. N. MALLER {Alternate) 
SHRI G, L, DUA 

SHRI S. K. SETHI {Alternate) 
SHRI R. C. KHANNA 

SHRI P. S. SAWHNEY {Alternate) 
MEMBER (HYDRO-ELECTRIC) 

DIRECTOR (HED)-I {Alternate) 
ER S. PANEERSELVAM 

SHRI V.JANARDHANAN {Alternate) 
SHRI K, P. R. PILLAI 

SHRI C. R. R. MEN ON {Alternate) 
SHRI V. RAD HA KRIS HN AN 
SHRI H. S. RAO 
PROF G. RAVEENDRAN NAiR 
SHRI S. R. SARDA 
SHRI R. SATHIYABAL 

SHRI K. K. MONDAL {Alternate) 
SHRI H. K. SITARAM 

SHRI S. K. PA LIT {Alternate) 
SHRI P. SRINIVASA POTI 

SHRI JOSEPH PHILOMENY {Alternate) 
SHRI D. S. TAW A RE 

SHRI SJ. HARIDAS {Alternate) 
SHRI G. N. THADANl 

SHRI S. K. GHOSH {Alternate) 
SHRI G, S. THAKUR 
SHRI V. T. WARANG 

SHRI R- P. PA TEL {Alternate) 
SHRI S. P. SACHDEV, 

Director (Elec tech) 



Jyoti Ltd, Vadodara 

Rural Electrification Corporation Ltd, New Delhi 

Delhi Electric Supply Undertaking, New Delhi 

Central Electricity Authority, New Delhi 

Tamil Nadu Electricity Board, Madras 

Fact Engineering and Design Organization, Udhyogamandal 

Bharat fleavy Electricals Ltd, Flyderabad 

Crompton Greaves Ltd, Bombay 

Chief Electrical Inspector to the Government of Kerala, Trivandrum 

Maharashtra State Electricity Board, Bombay 

Tariff Advisory Committee (General Insurance), Bombay 

Calcutta Electric Supply Corporation Ltd, Calcutta 

Karnataka Electricity Board, Bangalore 

Electrical Engineer to Government of Maharashtra, Bombay 

Engineers India Ltd, New Delhi 

Chief Electrical Inspector, Government of Madbya Pradesh, Bhopal 
Bombay Electric Supply and Transport Undertaking, Bombay 

Director General, BIS {Kx-ajficio Member) 

Secretary 
SHRI K, GANESH 
Deputy Director (Elec tech), BIS 

(Contimted on page 2) 



© Copyrfght 1 988 
BUREAU OF INDIAN STANDARDS 
This publication is protected under thQ Ine/ian Copyright Act (XIV 1957) and production in whole or in part by 
any meant except with written permission of the publisher shall be deemed to be an infringment of copyright 
under the said Act. 



13:3043-1987 



{Contimiecl from page 1 ) 



Panel for the Revision of IS : 3043, ETDC 20 : P38 



Convener 
SHRl N. BALASUBRAMANIAN 

Members 

I'ROF G. RAVEENDRAM NA!R 
SHRl V. SATHYANATHAN 
SHRl G. S. THARUR 
SHR] R. SATHIYABAL 
SHRl K. P. R. PILLAl 



Representing 
Larsen & Toubro (Construction Group), Madras 

Chief Electrical Inspector to the Government of Kerala, Trivandrum 

Tamil Nadu Electricity Board, Madras 

Chief Electrical Inspector, Government of Madhya Pradesh, Bhopal 

Tariff Advisory Committee, Madras 

Fact Engineering and Design Organization, Udyogamandal 



CONTENTS 



0. FOREWORD 

1. SCOPE 



IS : 3043 - 1987 



Page 

5 
6 



SECTION 1 GENERAL GUIDELINES 



2. TERMINOLOGY 

3. EXCHANGE OF INFORMATION 

4. STATUTORY PROVISIONS FOR EARTHING 

5. FACTORS INFLUENCING THE CHOICE OF 
SYSTEMS 

6. SYSTEM EARTHING 

7. EQUIPMENT EARTHING 



EARTHED AND UNEARTHED 



10 
11 
15 



SECTION 2 CONNECTIONS TO EARTH 

8. RESISTANCE TO EARTH ,,, 16 

9. EARTH ELECTRODE ... 19 

10. CURRENT DENSITY AT THE SURFACE OF AN EARTH ELECTRODE ... 27 

11. VOLTAGE GRADIENT AROUND EARTH ELECTRODES ... 27 

12. CONNECTIONS TO EARTH ELECTRODES — EARTHING AND PROTECTIVE 
CONDUCTORS ... 27 

13. EARTHING ARRANGEMENT FOR PROTECTIVE PURPOSES ... 32 

14. EARTHING ARRANGEMENT FOR FUNCTIONAL PURPOSES ... 32 

15. EARTHING ARRANGEMENTS FOR COMBINED PROTECTIVE AND 
FUNCTIONAL PURPOSES ... 32 

16. EQUIPOTENTIAL BONDING CONDUCTORS ... 33 

17. TYPICAL SCHEMATIC OF EARTHING AND PROTECTIVE CONDUCTORS ... 33 



SECTION 3 EARTH FAULT PROTECTION ON CONSUMER'S 

PREMISES 

18. EARTH FAULT PROTECTION IN INSTALLATIONS 

19. SELECTION OF DEVICES FOR AUTOMATIC DISCONNECTION OF SUPPLY 



34 

39 



SECTION 4 POWER STATIONS, SUBSTATIONS AND 
OVERHEAD LINES 

20. EARTHING IN POWER STATIONS AND SUBSTATIONS ... 43 

21. EARTHING ASSOCIATED WITH OVERHEAD POWER LINES ... 52 



SECTION 5 INDUSTRIAL PREMISES 

22. GUIDELINES FOR EARTHING IN INDUSTRIAL PREMISES 



53 



SECTION 6 STANDBY AND OTHER PRIVATE GENERATING 

PLANTS 

23. EARTHING IN STANDBY AND OTHER PRIVATE GENERATING PLANTS 

(INCLUDING PORTABLE AND MOBILE GENERATORS) ... 56 



IS : 3043 - 1987 

Page 

SECTION 7 MEDICAL ESTABUSHMENT 

24. PROTECTIVE MEASURES THROUGH EARTHTNG IN MEDICAL ESTABLISH- 
MENT 64 
25* SUPPLY CHARACTERISTICS AND PARAMETERS — 65 

SECTION 8 STATIC AND LIGHTNING PROTECTION EARTHING 

(Under consideration. Clauses 26 and 27 reserved for Section 8) 



SECTION 9 MISCELLANEOUS INSTALLATIONS AND 
CONSIDERATIONS 

28. EARTHING IN POTENTIALLY HAZARDOUS AREAS ••• 69 

29. TELECOMMUNICATION CIRCUITS AND APPARATUS ••• 70 

30. :BUILDING SITES ••• 71 

31. MINES AND QUARRIES 71 

32. STREET LIGHTING AND OTHER ELECTRICALLY SUPPLIES STREET 
FURNITURS 73 

33. EARTHING OF CONDUCTORS FOR SAFE WORKING ... 74 

34. MAINTENANCE OF EARTH ELECTRODES — 76 



SECTION 10 MEASUREMENTS AND CALCULATIONS 

35. CALCULATION OF EARTH FAULT CURRENTS 76 

36. MEASUREMENT OF EARTH RESISTIVITY ... 77 

37. MEASUREMENT OF EARTH ELECTRODE RESISTANCE ... 79 

38. MEASUREMENT OF EARTH LOOP IMPEDANCE ... 80 



SECTION 11 DATA PROCESSING INSTALLATIONS 

39. EARTHING REQUIREMENTS FOR INSTALLATIONS OF DATA PROCESSING 
EQUIPMENT 80 

40. EXAMPLE OF USE OF TRANSFORMERS ••• 83 



IS : 3043 - 1987 



Indian Standard 
CODE OF PRACTICE FOR EARTfflNG 

(First Revision) 

0. FOREWORD 



0.1 This Indian Standard (First Revision) was 
adopted by the Bureau of Indian Standards on 
6 August 1987, after the draft finalized by the 
Electrical Installations Sectional Committee, had 
been approved by the Electrotechnical Division 
Council. 

0.2 The Indian Electricity Rules, together with 
the supplementary regulations of the State Elec- 
tricity Departments and Electricity Undertakings, 
govern the electrical installation work in generat- 
ing stations, substations, industrial locations, 
buildings, etc, in the country. To ensure safety 
of life and apparatus against earth faults, it was 
felt necessary to prepare a code of practice for 
earthing. This code of practice is intended to 
serve as a consolidated guide to all those who 
are concerned with the design, installation, inspec- 
tion and maintenance of electrical systems and 
apparatus. 

0.3 The subject of earthing covers the problems 
relating to conduction of electricity through 
earth. The terms earth and earthing have been 
used in this code irrespective of reliance being 
placed on the earth itself as a low impedance 
return path of the foult current. As a matter of 
tact, the earth now rarely serves as a part of the 
return circuit but is being used mainly for 
fixing the voltage of system neutrals. The earth 
connection improves service continuity and 
avoids damage to equipment and danger to 
human life. 

0.4 The object of an earthing system is to provide 
as nearly as possible a surface under and around 
a station which shall be at a uniform potential 
and as nearly zero or absolute earth potential as 
possible. The purpose of this is to ensure that, in 
general, all parts of apparatus other than live 
parts, shall be at earth potential, as well as to 
ensure that operators and attendants shall be at 
earth potential at all times. Also by providing 
such an earth surl^ace of uniform potential under 
and surrounding the station, there can exist no 
difference of potential in a short distance big 
enough to shock or injure an attendant when 
short-circuits or other abnormal occurrences take 
place. The recommendations in this code are 
made in order that these objects may be carried 
out. 



0.5 Earthing associated with current-carrying 
conductor is normally essential to the security of 
the system and is generally known as system 
earthing, while earthing of non-current carrying 
metal work and conductor is essential to the safety 
of human life, animals and property, and is gene- 
rally known as equipment earthing. 

0.6 Since the publication ofthis standard in 1966, 
considerable experience has been gained through 
the implementation of its various stipulations. 
Moreover, several new concepts have been intro- 
duced the world over, on the understanding of 
functional and protective earthing with a view to 
take into account a variety of complex problems 
encountered in actual practice. In the context of 
increased use of electric power and the associated 
need for safety in the design of installations, it 
had become necessary to prepare an overall 
revision of the earlier version of the Code. 

0,7 In this Code, the terms 'earthing' and 
'grounding' are used synonymously. However, 
this Code introduces several new terms {see 2.15, 
2.17, 2.28, etc) and distinguishes earthing 'con- 
ductor' from 'protective conductor*. 

0.8 This Code includes comprehensive guidelines 
on choosing the proper size of the various com- 
ponents of the earthing system, particularly 
earthing and protective conductors as well as 
earth electrodes. Guidance included on determi- 
nation of relevant 7c' factor depending on (see 
Sec 2) material properties and boundary condi- 
tions, and the associated minimum cross-sectional 
area would assist in a more scientific design of the 
earthing system under various circumstances, 

0.9 For the first time, the Code also includes 
comprehensive guidelines on earth fault protec- 
tion in consumers' premises to commensurate 
with the provisions of IE Rules 1956. It includes 
specific guidelines on earthing system design to 
achieve the desired degree of shock hazard pro- 
tection from earth leakages. The rules given in 
Section 3 of the Code should be read in conjunc- 
tion with corresponding regulations given in the 
wiring code {see IS : 732). 

0.9.1 Protection against shock, both in normal 
service (direct contact) and in case of fault 
(indirect contact) can be achieved by several 



IS : 3043 - 1987 



measures. Details of such protective measures and 
guidance on their choice is the subject matter of 
debate in the process of revision of IS : 732''', 
Earth fault/leakage protection sought to be achie- 
ved through equipotential bonding and automatic 
disconnection of supply is envisaged to prevent a 
touch voltage from persisting for such a duration 
that would be harmful to human beings. Guid- 
ance on achieving this protection is' covered in 

Sec 3 ofthe Code. 

I 

0.9.2 While detailed guidelines arei covered in 

specific portions ofthe Code, the following shall 

be noted: 

I 

a) For solidly grounded systems, it shall be 
sufficient to check whether the, characteris- 
tics of protective device for automatic 
disconnection, earthing arrangements and 
relevant impedances of the j circuits are 
properly coordinated to ensure that voltages 
appearing between simultaneously accessi- 
ble, exposed and extraneous, conductive 
parts are within the magnitudes that would 
not cause danger; 

b) For systems where the earthing is deemed 
to be adequate, it shall be checked whether 
the main overcurrent protective device is 
capable of meeting the requirements in the 
wiring code; and 

c) Where the main overcurrent protective 
device did not fulfil the requirements or 
where the earthing is considered inade- 
quate, then a separate residual current 
device would be necessary to be installed, 
the earth fault loop impedance and the 
tripping characteristics so chosen that they 
comply with safe touch voltage limits. 

0.10 The revision ofthe Code aims at consolidat- 
ing in one volume all the essential guidelines 
needed for preparing a good earthing design in 
an electrical installation. The revision also 
attempts to be more elaborate than the earlier 
version, especially in areas of specific interest 
keeping in view the need and wide experience 
gained the world over. 



0.11 For convenience of identifying areas of inter- 
est by any specific users ofthe Code, the infor- 
mation contained in this standard is divided into 
different Sections as follows: 

Section 1 General guidelines; 

Section 2 Connections to earth; 

Section 3 Earth-fault protection in con- 
sumer's premises; 

Section 4 Power stations, substations and 
overhead lines; 

Section 5 Industrial premises; 

Section 6 Standby and other private gene- 
rating plant; 

Section 7 Medical establishments; 

Section 8 Static and lightning protection 
grounding; 

Section 9 Miscellaneous installations and 
considerations; 

Section 10 Measurements and calculations; 

and 
Section 11 Data processing installations. 

0.12 In the preparation of the Code, assistance 
has been taken from the following: 

lEC Pub 364 (and Parts) Electrical installa- 
tions in buildings. International Electro- 
technical Commission. 

BS Document 84/21243 Draft standard code 
of practice on earthing (revision ofCP 1013: 
1965). British Standards Institution. 

ANSI/IEEE Std 142-1982 IEEE Recommen- 
ded practice for grounding of industrial and 
commercial power systems. American 
National Standards Institute (USA). 

0.13 For the purpose of deciding whether a parti- 
cular requirement of this standard is complied 
with, the final value, observed or calculated, 
expressing the result of a test or analysis shall be 
rounded off in accordance with IS : 2-1960*. The 
number of significant places retained in the round- 
ed off value should be the same as that of the 
specified value in this standard. 



*Code of practice for electrical >viring installation. 



*Rules for rounding off numerical values ( revised). 



1. SCOPE 

1.1 This code of practice gives guidance on the 
methods that may be adopted to earth an electri- 
cal system for the purpose of limiting the potential 
(with respect to the general mass of the earth) 
of current carrying conductors forming part of 
the system, that is, system earthing and non- 



current carrying metal work association with 
equipment, apparatus and appliance connected 
to the system (that is, equipment earthing). 



1.2 This Code applies only to land-based installa- 
tions and it does not apply to ships, aircrafts or 
offshore installations. 



IS : 3043 - 1987 



SECTION 1 GENERAL GUIDELINES 



2. TERMINOLOGY 

2.0 For the purpose of this standard, the follow- 
ing definitions shall apply. 

2.1 Arc-Suppression Coil (Peterson Coil) — 

An earthing reactor so designed that its reac- 
tance is such that the reactive current to earth 
under fault conditions balances the capacitance 
current to earth flowing from the lines so that the 
earth current at the fault is limited to practically 
zero. 

2.2 Bonding Conductor — A protective con- 
ductor providing equipotential bonding. 

2.3 Class I Equipment — Equipment in which 
protection against electric shock does not rely on 
basic insulation only, but which includes means 
for the connection of exposed conductive parts to 
a protective conductor in the fixed wiring of the 
installation. 

NOTE — For inforrnation on classification of equip- 
ment with regard to means provided for protection 
against electric shock {see IS : 9409-1980*). 

2.4 Class n Equipment — Equipment in which 
protection against electric shock does not rely on 
basic insulation only, but in which additional 
safety precautions such as supplementary insula- 
tion are provided, there being no provision for 
the connection of exposed metalwork of the equip- 
ment to a protective conductor, and no reliance 
upon precautions to be taken in the fixed wiring 
of the installation. 

2.5 Dead — The term used to describe a device 
or circuit to indicate that a voltage is not applied. 

2.6 Double Insulation — Insulation comprising 
both basic and supplementary insulation. 

2.7 Earth — The conductive mass of the earth, 
whose electric potential at any point is conven- 
tionally taken as zero. 

2.8 Earth Electrode — A conductor or group of 
conductors in intimate contact with and providing 
an electrical connection to earth. 

2.9 Earth Electrode Resistance — The resis- 
tance of an earth electrode to earth, 

2.10 Earth Fault Loop Impedance — The 

impedance of the earth fault current loop (phase- 
to-earth loop) starting and ending at the point 
of earth fault. 

2.11 Earth Leakage Current — A current 
which flows to earth or to extraneous conductive 
parts in a circuit which is electrically sound. 

NOTE — This current may have a capacitive com- 
ponent including that resulting fix)m the deliberate use 
of capacitors. 



"^Classification of electrical and electronic equipment 
with regard to protection against electric shock. 



2.12 Earthed Concentric Wiring — A wiring 
system in which one or more insulated conductors 
are completely surrounded throughout their length 
by a conductor, for example, a sheath which acts 
as a PEN conductor. 

2.13 Earthing Conductor — A protective con- 
ductor connecting the main earthing terminal 
{see 2.2) (or the equipotential bonding conduc- 
tor of an installation when there is no earth bus) 
to an earth electrode or to other means of earth- 
ing. 

2.14 Electrically Independent Earth Electro- 
des — Earth electrodes located at such a distance 
from one another that the maximum current 
likely to flow through one of them does not signi- 
ficantly affect the potential of the other(s). 

2.15 Equipotential Bonding — Electrical con- 
nection putting various exposed conductive parts 
and extraneous conductive parts at a substantially 
equal potential. 

NOTE — In a building installation, equipotential 
bonding conductors shall interconnect the following 
conductive parts; 

a) Protective conductor; 

b) Earth continuity conductor; and 

c) Risers of air-conditioning systems and heating 
systems ( if any ). 

2.16 Exposed Conductive Part — A conductive 
part of equipment which can be touched and 
which is not a live part but which may become 
live under fault conditions. 

2.17 Extraneous Condctive Part — A conduc- 
tive part liable to transmit a potential including 
earth potential and not forming part of the elec- 
trical installation. 

2.18 Final Circuit — A circuit connected direc- 
tly to current-using equipment or to a socket 
outlet or socket outlets or other outlet points for 
the connection of such equipment. 

2.19 Functional Earthing — Connection to 
earth necessary for proper functioning of electrical 
equipment {see 29,1). 

2.20 Live Part — A conductor or conductive 
part intended to be energized in normal use 
including a neutral conductor but, by convention, 
not a PEN conductor. 

2.21 Main Earthing Terminal — The terminal 
or bar (which is the equipotential bonding con- 
ductor) provided for the connection of protective 
conductors and the conductors of functional ear- 
thing, ifany, to the means of earthing. 

2.22 Neutral Conductor — A conductor connec- 
ted to the neutral point of a system and capable 
of contributing to the transmission of electrical 
energy. 



IS : 3043 - 1987 



2.23 PEN Conductor — A conductor combin- 
ing the functions of both protective conductor and 
neutral conductor. 

2.24 Portable Equipment — Equipment which 
is moved while in operation or which can easily 
be moved from one place to another while conne- 
cted to the supply. 

2.25 Potential Gradient (At a Point) — The 

potential difference per unit length measured in 
the direction in which it is maximum. 

NOTE 1 — When an electric force is due to poten- 
tial difference, it is equal to the potential gradient. 

NOTE 2 — Potential gradient is expressed in volts 
per unit length. 

2.26 Protective Conductor — A conductor used 
as a measure of protection against electric shock 
and intended for connecting any of the following 
parts: 

a) Exposed conductive parts, 

b) Extraneous conductive parts, 

c) Main earthing terminal, and 

d) Earthed point of the source or an artificial 
neutral. 

2.27 Reinforced Insulation — Single insulation 
applied to live parts, which provides a degree of 
protection against electric shock equivalent to 
double insulation under the conditions specified in 
the relevant standard. 

NOTE — The term 'single insulation' does not imply 
that the insulation has to be one homogeneous piece. It 
may comprise several layers that cannot be tested singly 
as supplementary or basic insulation. 

2.28 Residual Current Device — A mechani- 
cal switching device or association of devices 
intended to cause the opening of the contacts 
when the residual current attains a given value 
under specified conditions. 

2.29 Residual Operating Current — Residual 
current which causes the residual current device 
to operate under specified conditions. 

2.30 Resistance Area (For an Earth Elec- 
trode only) — The surface area of ground 
(around an earth electrode) on which a signifi- 
cant voltage gradient may exist. 

2.31 Safety Extra Low Voltage ~~ See IS : 

9409-1980*. 

2.32 Simultaneously Accessible Parts — Con- 
ductors or conductive parts which can be touched 
simultaneously by a person or, where applicable, 
by livestock. 

NOTE 1 — Simultaneously accessible parts may be: 

a) live parts, 

b) exposed conductive parts, 

c) extraneous conductive parts, 

d) protective conductors, and 

e) earth electrodes. 



*Classification of electrical and electronic equipment 
with regard to protection against electrical shock. 



NOTE 2 — This term applies for livestock in loca- 
tions specifically intended tbr these animals. 

2.33 Supplementary Insulation — Independ- 
ent insulation applied in addition to basic insula- 
tion, in order to provide protection against 
electric shock in the event of a failure of basic 
insulation. 

2.34 Switchgear — An assembly of main and 
auxiliary switching apparatus for operation, regu- 
lation, protection or other control of electrical 
installations. 

NOTE — A more comprehensive definition of the 
term 'Switchgear' can be had from IS : 1885 (Part 
17)-I979*. 

2.35 Voltage, Nominal — Voltage by which an 
installation (or part of an installation) is desig- 
nated. 

2.36 Touch Voltage — The potential difference 
between a grounded metallic structure and a point 
on the earth's surface separated by a distance 
equal to the normal maximum horizontal reach, 
approximately one metre {see Fig. 1 ) . 

2.37 Step Voltage — The potential difference 
between two points on the earth's surface, separa- 
ted by distance of one pace, that will be assumed 
to be one metre in the direction of maximum 
potential gradient {see Fig. 1). 

2.38 Equipotential Line or Contour — The 

locus of points having the same potential at a 
given time. 

2.39 Mutual Resistance of Grounding Elect- 
rodes — Equal to the voltage change in one of 
them produced by a change of one ampere of 
direct current in the other and is expressed in 
ohms. 

2.40 Earth Grid — A system of grounding elec- 
trodes consisting of inter-connected connectors 
buried in the earth to provide a common ground 
for electrical devices and metalhc structures. 

NOTE — The term 'earth grid' does not include 
'earth mat'. 

2.41 Earth Mat — A grounding system formed 
by a grid of horizontally buried conductors and 
which serves to dissipate the earth fault current to 
earth and also as an equipotential bonding con- 
ductor system. 

3. EXCHANGE OF INFORMATION 

3.1 When the earthing of a consumer's installa- 
tion is being planned, prior consultation shall take 
place between the consultant or contractor and 
the supply authority. Where necessary, consulta- 
tions with the Posts & Telegraphs Department 
shall also be carried out in order to avoid any 
interference with the telecommunication system. 

*Electrotechnical vocabulary: Part 17 Switchgear and 
controlgear (first revision). 



IS : 3043 - 1987 





RISE ABOVE REMOTE 
INC SHORT CIRCUIT 



STEP VOLTAGE AT A GROUNDED STRUCTURE^ 



E TOUCH 





OTENTIAL RISE ABOVE REMOTE 
EARTH DURING SHORT CIRCUIT 



yvrrrrrr' 



TOUQI VOLTAGE AT A GROUNDED STRUCTURE 
FIG. 1 STEP AND TOUCH VOLTAGES 



4. STATUTORY PROVISIONS FOR 
EARTHING 

4.1 Earting shall generally be carried out in 
accordance with the requirements of Indian Electri- 
city Rules 1956, as amended from time to time and 
the relevant regulations of the Electricity Supply 
Authority concerned. 

4.2 All medium voltage equipment shall be 
earthed by two separate and distinct connections 
with earth. In the case of high and extra high 
voltages, the neutral points shall be earthed by 
not less than two separate and distinct connec- 
tions with earth, each having its own electrode at 
the generating station or substation and may be 
earthed at any other point provided no interfer- 
ence is caused by such earthing, if necessary, the 
neutral may be earthed through a suitable 
impedance. 

4.2.1 In cases where direct earthing may prove 
harmful rather than provide safety (for example, 
high frequency and mains frequency coreless 
induction furnaces), relaxation may be obtained 
from the competent authority. 



4.3 Earth electrodes shall be provided at generat- 
ing stations, substations and consumer premises 
in accordance with the requirements of this 
Code. 

4.4 As far as possible, all earth connections shall 
be visible for inspection. 

4.5 All connections shall be carefully made; if 
they are poorly made or inadequate for the pur- 
pose tbr which they are intended, loss of life or 
serious personal injury may result. 

4.6 Each earth system shall be so devised that the 
testing of individual earth electrode is possible. It 
is recommended that the value of any earth 
system resistance shall be such as to conform with 
the degree of shock protection desired. 

4.7 It is recommended that a drawing showing 
the main earth connection and earth electrodes be 
prepared for each installation. 

4.8 No addition to the current-carrying system, 
either temporary or permanent, shall be made 
which will increase the maximum available earth 



IS : 3043 - 1987 



fault current or its duration until it has been 
ascertained that the existing arrangement of earth 
electrodes, earth bus-bar, etc, are capable of 
carrying the new value of earth fault current 
which may be obtained by this addition. 

4.9 No cut-out, link or switch other than a linked 
switch arranged to operate simultaneously on the 
earthed or earthed neutral conductor and the live 
conductors, shall be inserted on any supply 
system. This, however, does not include the case 
of a switch Ibr use in controlling a generator or 
a transformer or a link for test purposes. 

4.10 All materials, fittings, etc, used in earthing 
shall conform to Indian Standard specifications, 
wherever these exist. 

5 . FACTORS INFLUENCING THE CHOICE 
OF EARTHED OR UNEARTHED 
SYSTEM 

5.1 Service Continuity 

5.1.1 A number of industrial plant systems have 
been operated unearthed at one or more voltage 
levels. This is basically guided by the thought of 
gaining an additional degree of service continuity 
varying in its importance depending on the type 
of plant. Earthed systems are in most cases desi- 
gned so that circuit protective devices will remove 
the faulty circuit from the system regardless of 
the type of fault. However, experience has shown 
that in a number of systems, greater service conti- 
nuity may be obtained with earthed-neutral than 
with unearthed neutral systems. 

5.2 Multiple Faults to Ground 

5.2.1 While a ground fault on one phase of an 
unearthed system generally does not cause a 
service interruption, the occurrence of a second 
ground fault on a different phase before the first 
fault is cleared, does result in an outage. The 
longer a ground fault is allowed to remain on an 
unearthed system, greater is the likelihood of a 
second one occurring in another phase and repairs 
are required to restore service. With an unear- 
thed system, an organized maintenance pro- 
gramme is therefore extremely important so that 
faults are located and removed soon after detec- 
tion. 

Experience has shown that multiple ground 
faults are rarely, if ever, experienced on earthed- 
neutral systems. 

5.3 Arcing Fault Burndowns 

5,3,1 In typical cases, an arcing fault becomes 
established between two or more phase conductors 
in an unearthed systems or between phase and 
ground in a solidly earthed-neutral system. This 
would result in severe damage or destruction to 
equipment. However, arcing fault current levels 
may be so low that phase overcurrent protective 
devices do not operate to remove the fault quickly. 
Such faults are characteristic of open or covered 
fuses, particularly in switchgear or metal-enclosed 



switching and motor control equipment. It is 
generally recognized that protection under such 
circumstances is possible by fast and sensitive 
detection of the arcing fault current and interrup- 
tion within 10-20 cycles. In solidly earthed- 
neutral systems, this is possible as an arcing fault 
would produce a current in the ground path, 
thereby providing an easy means of detection and 
tripping against phase-to-ground arcing fault 
breakdowns. 

5.4 Location of Faults 

5.4.1 On an unearthed system, a ground fault 
does not open the circuit. Some means of detect- 
ing the presence of a ground fault requires to be 
installed. In earthed system, an accidental ground 
fault is both indicated at least partially located 
by an automatic interruption of the accidentally 
grounded circuit or piece of equipment. 

5.5 Safety 

5.5.1 Whether or not a system is grounded, 
protection of personnel and property from hazards 
require thorough grounding of equipment and 
structures. Proper grounding results in less likeli- 
hood of accidents to personnel. Other hazards of 
shock and fire may result from inadequate groun- 
ding of equipment in unearthed and earthed 
systems. However, relatively high fault currents 
associated with solidly earthed system may pre- 
sent a hazard to workers from exposure to hot 
arc products and flying molten metal. This pro- 
tection is, however, reduced because of use of 
metal-enclosed equipment. 

5.6 Abnormal Voltage Hazards 

5.6.1 The possible over-voltages on the unear- 
thed system may cause more frequent failures of 
equipment than is the system, if earthed. A fault 
on one phase of an unearthed or impedance- 
grounded system places a sustained increased 
voltage on the insulation of ungrounded phases 
in a 3-phase system. This voltage is about 1.73 
times the normal voltage on the insulation. This 
or other sustained over-voltages on the unearthed 
system may not immediately cause failure of 
insulation but may tend to reduce the life of the 
insulation. Some of the more common sources of 
over-voltages on a power system are the follow- 
ing; 

a) Lightning, 

b) Switching surges, 

c) Static, 

d) Contact with a high voltage system, 

e) Line-to-ground fault, 

f) Resonant conditions, and 

g) Restriking ground faults. 

5.6.2 Surge arresters are recommended for 
lightning protection. Grounding under such cases 
are separately discussed in Section 8. Neutral 



10 



IS : 3043 - 1987 



grounding is not likely to reduce the total magni- 
tude of over-voltage produced by lightning or 
switching surges. It can, however, distribute the 
voltage between phases and reduce the possibility 
of excessive voltage stress on the phase-to-ground 
insulation of a particular phase. A system ground 
connection even of relatively high resistance can 
effectively prevent static voltage build-up (see 
Sec 8). Even under conditions of an HV line 
breaking and falling on an LV system, an effecti- 
vely grounded LV system will hold the system 
neutral close to the ground potential thus limiting 
the over-voltage. An unearthed system will be 
subjected to resonant over-voltages. Field experi- 
ence and theoretical studies have shown the world 
over that arcing, restriking or vibrating ground 
faults on unearthed systems can, under certain 
conditions, produce surge voltages as high as 6 
times the normal voltage. Neutral grounding is 
effective in reducing transient build up by reduc- 
ing the neutral displacement from ground poten- 
tial and the destructiveness of any high frequency 
voltage oscillations following each arc initiation 
or restrike. 

5.7 Cost 

5.7.1 The cost differential between earthed 
and unearthed neutral system will vary, depend- 
ing on the method of grounding the degree of 
protection desired, and whether a new or an 
existing system is to be earthed. 

6. SYSTEiM EARTHING 

6.0 Basic Objectives 

6.0.1 Earthing of system is designed primarily 
to preserve the security of the system by ensuring 
that the potential on each conductor is restricted 
to such a value as is consistent with the level of 
insulation applied. From the point of view of 
safety, it is equally important that earthing should 
ensure efficient and fast operation of protective 
gear in the case of earth faults. Most high voltage 
public supply systems are earthed. Approval has 
been given in recent years to unearthed overhead 
line systems in certain countries, but these have 
only been small 11 kV systems derived from 
33 kV mains, where the capacity earth current is 
less than 4 A and circumstances are such that the 
system will not be appreciably extended. 

6.0.2 The limitation of earthing to one point 
on each system is designed to prevent the passage 
of current through the earth under normal condi- 
tions, and thus to avoid the accompanying risks 
of electrolysis and interference with communica- 
tion circuits. With a suitable designed system, 
properly operated and maintained, earthing at 
several points may be permitted. This method of 
earthing becomes economically essential in sys- 
tems at 200 kV and upwards. 

6,0.3 The system earth-resistance should be 
such that, when any fault occurs against which 



earthing is designed to give protection, the pro- 
tective gear will operate to make the faulty main 
or plant harmless. In most cases, such operation 
involves isolation of the faulty main or plant, for 
example, by circuit-breakers or fuses. 

6.0.4 In the case of underground systems, there 
is no difficulty whatever but, for example, in the 
case of overhead-line systems protected by fuses 
or circuit-breakers fitted with overcurrent protec- 
tion only, there may be difficulty in arranging 
that the value of the system earth-resistance is 
such that a conductor falling and making good 
contact with the ground results in operation of 
the protection. A low system-earth resistance 
is required even in the cases where an arc- 
suppression coil is installed, as its operation may 
be frustrated by too high an earth-electrode 
resistance. 

6.0.5 Earthing may not give protection against 
faults that are not essentially earth faults. For 
example, if a phase conductor on an overhead spur 
line breaks, and the part remote from the supply 
falls to the ground, it is unlikely that any protec- 
tive gear relying on earthing, other than current 
balance protection at the substation, will operate 
since the earth-fault current circuit includes the 
impedance of the load that would be high relative 
to the rest of the circuit. 

6,0.6 For the purposes of this code of practice, 
it is convenient to consider a system as comprising 
a source of energy and an installation; the former 
including the supply cables to the latter. 



6.1 Classification of Systems 
Types of System Earthing 



Based on 



6,1.1 Internationally, it has been agreed to 
classify the earthing systems as 77V System, TT 
System and IT System. They are: 

a) TN system — has one or more points of the 
source of energy directly earthed, and the 
exposed and extraneous conductive parts 
of the installation are connected by means 
of protective conductors to the earthed 
point(s) of the source, that is, there is a 
metallic path for earth fault currents to 
flow from the installation to the earthed 
point(s) of the source. TN systems are 
further sub-divided into TN-C, TN-S and 
TN-C-S systems. 

b) TT system — has one or more points of the 
source of energy directly earthed and the 
exposed and extraneous conductive parts 
of the installation are connected to a local 
earth electrode or electrodes are electri- 
cally independent of the source earth(s). 

c) IT system — has the source either unear- 
thed or earthed through a high impedance 
and the exposed conductive parts of the 
installation are connected to electrically 
independent earth electrodes. 



11 



IS : 3043 - 1987 



6.1.2 It is also recognized that, in practice, a 
system may be an admixture of type for the pur- 
poses of this code, earthing systems are designated 
as follows: 

a) TN-S System (for 240 V single phase domestic/ 
commercial supply) — Systems where there 
are separate neutral and protective con- 
ductors throughout the system. A system 
where the metallic path between the 
installation and the source of energy is the 
sheath and armouring of the supply cable 
{see Fig. 2). 

b) Indian TN-S System (for 415 V three-phase 
domestic commercial supply) ~ An independ- 
ent earth electrode within the consumer's 
premises is necessary (See Fig. 3). 

Indian TN C-System — The neutral and pro- 
tective functons are combined in a single 

SOURCE OF eWERGY 



couductor throughout the system (for 
example earthed concentric wiring (see 
Fig. 4). 

d) TN-C-S System — The neutral and protec- 
tive functions are combined in a single 
conductor but only in part of the system 
(see Fig 5 ) . 

e) T-TN-S System (for 6*6/11 kV three-phase 
hulk supply) — -The consumers installation, 
a TN-S system receiving power at a captive 
substation through a delta connected 
transformer primary (see¥\g. 6). 

f) TT System for 415V three-phase industrial 
supply) — Same as 6.1.1 (b) (see Fig 7.). 



c) 



g) IT System 
Fig. 8 ). 



Same as 6.1.1 (c) ( see 




EQUIPMENT IN 
INSTALLATION 



CONDUCTIVE 
PA«T 



CONSUMER 



I ! INSTALLATION 



FIG. 2 



NOTE — The protective conductor (PE) is the metallic covering (armour or load sheath of the cable supplying 
the installation or a separate conductor). 

All exposed conductive parts of an installation are connected to this protective conductor via main earthing 
terminal of the installation. 

TM-S SYSTEM SEPARATE NEUTRy\L AND PROTECTIVE CONDUCTORS THROUGHOUT THE SYSTEM, 
230V SIMPLE PHASE. DOMESTIC/COMMERCIAL SUPPLY FOR 3~TN-S (See FIG. 3) 

SOURCE OF ENERGY 

L2 
L3 



^ 
















1 ' — — 




_ ... ,.,. ^'m...m -K«— - _ 


( — 







PE 



415 V Three phase Domestic/Commercial supply having 3 ~ and 



loads. 



All exposed conductive parts of the installation are connected to protective conductor via the main earthing 
terminal of the installation. An independent earth electrode within the consumer's permises is necessary. 

FIG. 3 INDIAN TN-S SYSTEM 

12 



JS : 3043 - 1987 



S0UI9CE OF £NCR6V 

cz>— 



Z3- 



source 

CARTH 



06^ 



66 



V<T^ 



3 ^CONSUME 



5UM£fi/ \£XP< 



J L 




PO SEO CONOUCTIVE 
PARTS 



— U 

-12 

-COMSJNeO 
P£ & N 

CONDUCTOR 
S AOOITIQNAl 
I SOURCE EARTH 
j (MAY BE PROVrOEO) 



T 



\. 



iNSTALLATiON 



AH exposed conductive parts are connected to the PEN conductor. For 3 
to be provided in addition. 



consumer, local earth electrode has 



FIG. 4 [NDTAN TN-C SYSTEM (NEUTRAL AND PROTECTIVE FUNCTIONS COMBINED IN 
A SINGLE CONDUCTOR THROUGHOUT SYSTEM ) 



•LI 
-13 



06600 



L *J 



;t 



— COMBfNEO 

PE a. N 

CONDUCTHR 



The usual form of a TN-C-S system is as shown, where the supply is TN-C and the arrangement in the 
installations in TN-S. 

This type of distribution is known also as Protective Multiple Earthing and the PEN conductor is referred to as 
the combined neutral and earth (CNE) Conductor. 

The supply system PEN conductor is earthed at several points and an earth electrode may be necessary at or 
near a consumer's installation. 

All exposed conductive parts of an installation are connected to the PEN conductor via the main earthing 
terminal and the neutral terminal, these terminals being linked together. 

The protective neutral bonding (PNB) is a variant of TN-C-S with single point earthing. 

FIG. 5 TiN-C-S SYSTEM, NEUTRAL AND PROTECTIVE FUNCTIONS COMBINED IN A SINGLE 
CONDUCTOR IN A PART OF THE SYSTEM 



13 



IS : 3043 - 1987 



SOU^C 0? €HtMy 



r 



CONSUME f« 
9NSTAIUTS0N 



LAJ 



L.-.. 



Kyi . 



«L3 



f*"4 6 d o 

3w^ lOAO 



] 



CONSUMER 
iNSTAUATION 



* 



ir~7i 



1«10A0 

— t 



6 O 
3 v^ LOAD 



• 1 
-2 
-3 



6*6/1 1 kV Three phase bulk supply. 
FIG. 6 T-TN-S SYSTEM 



SOURCE or ENERGY 



.As. 



SOURCE 
EARTH 



CONSUMER I 
IHSTAllATION | 



u 



J 



X 



9 9 



-13 



6 O 



INSTAllAtlON 

EARTH 
ELECTROOE 



415 V Three phase industrial supply having 3 - and 1 ~ loads. 

All exposed conductive parts of the installation are connected to an earth electrode which is electrically inde- 
pendent of the source earth. Single phase TT system not present in India. 

FIG. 7 TT SYSTEM 



SOURCE OF 
EHERGY 




All exposed conductive parts of an installation are connected to an earth electrode. 

The source is either connected to earth through a deliberately introduced earthing impedance or is isolated 
trom earth. 



FIG. 8 



IT SYSTEM 
14 



IS : 3043 - 1987 



6.2 Marking of Earthing/Protective Conduc- 
tor 

6.2.1 The earthing and protective conductor 
shall be marked as given in Table 1 {see also 
IS: 11353-1986*). 



TABLE 1 MARKING OF CONDUCTORS 


DESIGNA- 


IDENTIFICATION BY 


COLOUR 


TION Of 


r~— *— "* 


■— ~ -^ 




CONDUCTOR 


Alphanu- 


Graphical 






meric 


Syinbol 






Notation 






Earth 


E 


i 


No colour other 






than colour of 








the bare con- 








ductor 


Protective 


PB 




Green and yellow 


conductor 









6.2.2 Use of Bi-Coloiir Combination — Green and 
Yellow — The bi-colour combination, green and 
yellow (green/yellow), shall be used for identify- 
ing the protective conductor and for no other 
purpose. This is the only colour code recognized 
for identifying the protective conductor. 

Bare conductors or busbars, used as protective 
conductors, shall be coloured by equally broad 
green and yellow stripes, each 15 mm up to 100 
mm wide, close together, either throughout the 
length of each conductor or in each compartment 
or unit or at each accessible position. If adhesive 
tape is used, only bi-coloured tape shall be 
applied. 

For insulated conductors, the combination of 
the colours, green and yellow, shall be such that, 
on any 15 mm length of insulated conductor, one 
of these colours covers at least 30 percent and 
not more than 70 percent of the surface of the 
conductor, the other colour covering the remain- 
der of that surface. 

NOTE — Where the protective conductor can hQ 
easily identified from its shape, construction or position, 
for example, a concentric conductor, then colour 
coding throughout its length is not necessary but the 
ends or accessible positions should be clearly identified 
by a symbol or the bi-colour combination, green and 
yellow, 

7. EQUIPMENT EARTHING 
7.0 Basic Objectives 

7.0.1 The basic objectives of equipment 
grounding are: 

i) to ensure freedom from dangerous electric 



■-^Guide for uniform system of marking and identifica- 
tion of conductors and apparatus terminals. 



sliock voltages exposure to persons in the 
area; 

2) to provide current carrying capability, both 
in magnitude and duration, adequate to 
accept the ground fault current permitted 
by the overcurrent protective system with- 
out creating a fire or explosive hazard to 
building or contents; and 

3) to contribute to better performance of the 
electrical system. 

7.0.2 Voltage Exposure — When there is un- 
intentional contact between an energized electric 
conductor and the metal frame or structure that 
encloses it (or is adjacent, the frame or structure 
tends to become energized to the same voltage 
level as exists on the energized conductor. To 
avoid this appearance of this dangerous, exposed 
shock hazard voltage, the equipment grounding 
conductor must present a low impedance path 
from the stricken frame to the zero potential 
ground junction. The impedance should also be 
sufficiently low enough to accept the hall magni- 
tude of the line-to-ground fault current without 
creating an impedance voltage drop large enough 
to be dangerous. 

7.0.3 Avoidance of Thermal Distress — The 
earthing conductor must also function to conduct 
the full ground fault current (both magnitude 
and duration) without excessively raising the 
temperature of the earthing conductor or causing 
the expulsion of arcs and sparks that could initiate 
a fire or explosion. The total impedance of the 
fault circuit including the grounding conductor 
should also permit the required current amplitude 
to cause operation of the protective system. 

7.0.4 Preservation of System Performance — The 
earthing conductor must return the ground fault 
current on a circuit without introducing enough 
additional impedance to an extent that would 
impair the operating performance of the overcurr-^ 
ent pi^otective device, that is, a higher than 
necessary ground-circuit impedance would be 
acceptable if there is no impairment of the per- 
formance characteristics of the protective system. 

7.1 Classification ofEquipment with Regard 
to Protection Against Electric Shock 

7.1.1 Table 2 gives the principal characteris- 
tics of equipment according to this classification 
and indicates the precautions necessary for safety 
in the event of failure of the basic insulation. 



TABLE 2 CLASSIFICATION OF EQUIPMENT 



Principal 
characteristics 
of equipment 

Precautions for 
safety 



CLASS 
No means of 
protective 
earthing 

Earth free 
environment 



CLASH 1 
Protective 
earthing means 
provided 

Connection to 
the protective 
earthing 



CLASS II 
Additional insula- 
tion and no means 
for protective 
carting 

None necessary 



CLASS III 
Designed for supply 
at safety extra 
low voltage 

Connection to safety 
extra low voltage 



15 



IS : 3043 - 1987 



SECTION 2 CONNECTIONS TO EARTH 



8. RESISTANCE TO EARTH 

8.0 Nature of Earthing Resistance 

8.0.1 The earthing resistance of an electrode is 
made up of: 

a) resistance of the (metal) electrode, 

b) contact resistance between the electrode 
and the soil, and 

c) resistance of the soil from the electrode sur- 
face outward in the geometry set up for the 
flow of current outward from the electrode 
to infinite earth. 

The first two factors are very small fractions of 
an ohm and can be neglected tbr all practical pur-^ 
poses. The factor of soil resistivity is discussed 
in 8.1. 

8.1 Soil Resistivity 

8.1.1 The resistance to earth of a given elec- 
trode depends upon the electrical resistivity of 
the soil in which it is installed. This factor is, 
therefore, important in deciding which of many 
protective systems to adopt. 

The type of soil largely determines its resisti- 
vity and examples are given in Table 3. Earth 
conductivity is, however, essentially electrolytic 
in nature and is affected, by the moisture content 
of the soil and by the chemical composition and 



concentration of salts dissolved in the contained 
water. Grain size and distribution, and closeness 
of packing are also contributory factors since they 
control the manner in which the moisture is held 
in the soil. Many of these factors vary locally and 
some seasonally so that the table should only be 
taken as a general guide. 

Local values should be verified by actual mea- 
surement, and this is especially important where 
the soil is stratified as, owing to the dispersion of 
the earth current, the effective resistivity depends 
not only on the surface layers but also on the 
underlying geological formation. 

It should also be noted that soil temperature 
has some effect (see 8.7), but is only important 
near and below freezing point, necessitating the 
installation of earth electrodes at depths to which 
frost will not penetrate. It is, therefore, recom- 
mended that the first metre of any earth electrode 
should not be regarded as being effective under 
frost conditions. 

While the fundamental nature and properties 
of a soil in a given area cannot be changed, use 
can be made of purely local conditions in choosing 
suitable electrode sites and methods of preparing 
the site selected to secure the optimum resistivity. 
These measures may be summarized as in 8,2 
to 8.7. 



TYPB OF SOIL 



TABLE 3 EXAMPLES OF SOIL RESISTIVITY 



CLIMATIC CONDITION 





Normal an 

Rainfall 
Example, 
than 500 mm 


d High 

(for 

Greater 

a Year 


Low Rainfall and 
Desert Condition (For 
Examples, Less than 
) 250 mm a Year) 


Underground 

Waters 

(Salids) 




Probable 
value 




Range of 

values 
encountered 


Range of 

values 

encountered 


Range of 

values 

encountered 


(1) 


(2) 




(3) 

a.m 


(4) 

a.m 


(5) 

a.m 


Alluvium and lighter clays 


5 




* 


^ 


1 to 5 


Clays (excluding alluvium) 


10 




5 to 20 


10 to 100 




Marls (for example, keuper marl) 


20 




10 to 30 


50 to 300 




Porous limestone (for example, chalk) 


50 




30 to 100 






Porous sandstone (for example, keuper 
sandstone and clay shales) 


100 




30 to 300 






Quartzites, compact and crystalline 
limestone (for example, carbonife- 
rous marble, etc) 


300 




100 to 1 000 






Clay slates and slatey shales 


1 000 




300 to 3 000 


1 000 upwards 


30 to 100 


Granite 


1 000 










Fossile slates, schists gneiss igneous 
rocks 


2 000 




1 000 upwards 






*Depends on water level of locality. 













16 



18:3043-1987 



8.2 Where there is any option, a site should be 
chosen in one of the following types of soil in the 
order of preference given: 

a) Wet marshy ground {see 8.3); 

b) Clay, loamy soil, arable land, clayey soil, 
clayey soil or loam mixed with small quan- 
tities of sand; 

c) Clay and loam mixed with varying propor- 
tions of sand, gravel and stones; 

d) Damp and wet sand, peat. 

Dry sand, gravel chalk, limestone, granite and 
any very stony ground should be avoided, and also 
all locations where virgin rock is very close to the 
surface. 

8.3 A site should be chosen that is not naturally 
well-drained. A water-logged situation is not, 
however, essential, unless the soil is sand or gra- 
vel, as in general no advantage results from an 
increase in moisture content above about 15 to 20 
percent. Care should be taken to avoid a site kept 
moist by water flowing over it (for example, the 
bed of a stream) as the beneficial salts may be 
entirely removed from the soil in such situations. 

8.4 Where building has taken place, the site con- 
ditions may be altered by disturbance of the local 
stratification and natural geological formation 
when the electrodes have to be installed in this 
disturbed area. 

If a cut and fill exercise has been carried out 
then the top layer may be similar to the natural 
formation but increased in depth, whether it is 
good or bad in terms of resistivity. 

If an imported fill exercise has been carried 
out, the conditions of the upper layers may be 
altered considerably. 

In these cases, deeper driving of the electrode 
may be necessary to reach layers of reasonable 
resistivity and also to reach stable ground, such 
that the value of the electrode resistance remains 
stable if the top layers of the ground dry out. 

8.5 Soil treatment to improve earth electrode con- 
tact resistance may be applied in special or diffi- 
cult locations, but migration and leaching of 
applied chemicals over a period oftime reduces 
the efficiency of the system progressively, requiring 
constant monitoring and replacement ofthe addi- 
tives. Ecological considerations are inherent be- 
fore such treatment is commenced and any dele- 
terious effect upon electrode material has to be 
taken into account. However, for some temporary 
electrical installations in areas ofhigh ground resis- 
tivity, this may be the most economic method for 
obtaining satisfactory earth contact over a short 
period of working. If a greater degree of perman- 
ence is envisaged, earth electrodes packaged in 
material such as bentonite are preferable. 

Bentonite or similar material may be used to 
advantage in rocky terrain. Where holes are bored 



for the insertion of vertical electrodes or where 
strip electrodes are laid radially under shallow 
areas of low resistivity overlaying rock strata, ben-^ 
tonite packing will increase the contact efficiency 
with the general mass of ground. 

8.6 Effect of Moisture Content on Earth 
Resistivity — Moisture content is one of the 
controlling factors in earth resistivity. Figure 9 
shows the variation of resistivity of red clay soil 
with percentage of moisture. The moisture content 
is expressed in percent by weight ofthe dry soil. 
Dry earth weighs about 1 440 kg per cubic metre 
and thus 10 percent moisture content is equivalent 
to 144 kg of water per cubic metre of dry soil. It 
will be seen from Fig. 9 that above about 20 per- 
cent moisture, the resistivity is very little affected, 
while below 20 percent the resistivity increases 
very abruptly with the decrease in moisture con- 
tent. A difference of a few percent moisture will 
therefore, make a very marked difference in the 
effectiveness of earth connection if the moisture 
content falls below 20 percent. The normal mois- 
ture content of soils ranges from 10 percent in dry 
seasons to 35 percent in wet seasons, and an ap- 
proximate average may be perhaps 16 to 18 per- 
cent. 

It should be recognized, however, that mois- 
ture alone is not the predominant factor in the low 
resistivity of soils; for example, earth electrodes 
driven directly in the beds of rivers or mountain 
streams may present very high resistance to earth. 
If the water is relatively pure, it will be high resis- 
tivity and unless the soil contains sufficient 
natural elements to form a conducting electrolyte, 
the abundance of water will not provide the soil 
with adequate conductivity. The value of high 
moisture content in soils is advantageous in increas- 
ing the solubility of existing natural elements in 
the soil, and in providing for the solubility of in- 
gredients which may be artificially introduced to 
improve the soil conductivity. 

8.7 Effect of Temperature on Earth Resis- 
tance — The temperature coefficient of resistivity 
for soil is negative, but is negligible for tempera- 
tures above freezing point. At about 20°C, the 
resistivity change is about 9 percent per degree 
Celsius. Below 0°C the water in the soil begins to 
freeze and introduces a tremendous increase in the 
temperature coefficient, so that as the temperature 
becomes lower the resistivity rises enormously. It 
is, therefore, recommended that in areas where 
the temperature is expected to be quite low, the 
earth electrodes should be installed well below the 
frost line. Where winter seasons are severe, this 
may be about 2 metres below the surface, whereas 
in mild climates the frost may penetrate only a 
few centimetres or perhaps the ground may not 
freeze at all. Earth electrodes which are not driven 
below the first depth may have a very great vari- 
ation in resistance throughout the seasons ofthe 
year. Even when driven below the frost line, there 
is some variation, because the upper soil, when 



17 



IS : 3043 - 1987 



frozen, presents a decided increase in soil resisti- 
vity and lias the effect of shortening the active 
length of electrode in contact with soil of normal 
resistivity. 

8.8 Artificial Treatment ofSoil — Multiple 
rods, even in large number, may sometime fail to 

produce an adequately low resistance to earth. 

This condition arises in installations involving 
soils of high resistivity. The alternative is to reduce 
the resistivity of the soil immediately surrounding 
the earth electrode. To reduce the soil resistivity, 
it is necessary to dissolve in the moisture, norm- 
ally contained in the soil, some substance which is 
highly conductive in its water solution. The most 
commonly used substances are sodium chloride 
(NaCl), also known as common salt, calcium 
chloride (CaCh), sodium carbonate (Na2C03), 
copper sulphate (CUSO4), salt, and soft coke, and 
salt and charcoal in suitable proportions. 



8.8.1 With average or high moisture content, 
these agents form a conducting electrolyte through- 
out a wide region surrounding the earth elec- 
trode. Approximately 90 percent of the resistance 
between a driven rod and earth lies within a radi- 
us of about two metres from the rod. This should 
be kept in mind when applying the agents for 
artificial treatment of soil. The simplest applica- 
tion is by excavating a shallow basin around the 
top of the rod, one metre in diameter and about 
30 cm deep, and applying the artificial agent in 
this basin. The basin should subsequently be filled 
several times with water, which should be allowed 
each time to soak into the ground, thus carrying 
the artificial treatment, in electrolyte form, to con- 
siderable depths and allowing the artificial agent 
to become diffused throughout the greater part 
of the effective cylinder of earth surrounding the 
driven rod. 






i 






> 





i_ j :: r_ i_ :^ _ _ . : 


1 _ 




























, 














~ 1 + 


















— . « — — •« — 








.L 


J 


5<=^ " 



5 » IS 20 2S 30 35 40 45 50 55 60 $5 70 
MOISTURE IN SOIL,P£ftC£NT 



FIG. 9 VARIATION OF SOIL RESISTIVITY WITH MOISTURE CONTENT 



18 



IS : 3043 - 1987 



8,8.2 The reduction in soil resistivity effected 
by salt is shown by the curve in Fig. 10. The salt 
content is expressed in percent by weight of the 
contained moisture. It will be noted that the 
curve flattens off at about 5 percent salt content and 
a further increase in salt gives but little decrease 
in the soil resistivity. The effect of salt will be 
different for different kinds of soil and for various 
moisture contents but the curve will convey an 
idea of how the soil conductivity can be impro- 
ved. Decreasing the soil resistivity causes a corres- 
ponding decrease in the resistance of a driven 
earth electrode. 



40d 




































jg no 

It son 




































1 ^ 




































O 290 




































g m 




































2 UUI 


i«ii' 


































5 "• 


\ 




































^ 


V. 




■M 




^^ 


_ 




_ 




^ 













FIG. 



2 4 e • Id 12 1« Sd 

^f nccMT or SAa m moistuae 

10 VARIATION OF SOIL RESISTIVITY WITH 
SALT (Nacl) CONTENT, CLAY SolL 
HAVING 3 PERCENT MOISTURE 



8.8.3 In close texture soils, the artificial treat- 
ment may be effective over a period of many years. 
However, it is recommended that annual or bi- 
annual measurements of earth resistivity should 
be made to find out if additional treatment is 
needed. 

8.8.4 In using artificial treatment, the possible 
corrosive effect of the salt on the driven rods and 
connections should be considered. The possible 
contamination of the domestic water supply should 
also be considered. 

9. EARTH ELECTRODES 

9.1 Effect of Shape on Electrode Resistance 

9.1.1 With all electrodes other than extended 
systems, the greater part of the fall in potential 
occurs in the soil within a few feet of the electrode 
surface, since it is here that the current density is 
highest. To obtain a low overall resistance the 
current density should be as low as possible in the 
medium adjacent to the electrode, which should 
be so designed as to cause the current density to 
decrease rapidly with distance from the electrode. 
This requirement is met by making the dimensi- 
ons in one direction large compared with those in 



the other two, thus a pipe, rod or strip has a much 
lower resistance than a plate of equal surface area. 
The resistance is not, however, inversely propor- 
tional to the surtiice area of the electrode. 

9.2 Resistance of Common Types of Earth- 
Electrodes 

9.2.1 Plates — The approximate resistance to 
earth of a plate can be calculated from: 



^•"TV x""^* 



where 

p = resistivity of the soil (assumed uni- 
form) (in n.m); and 

A = area of both sides of the plate (in m^). 

Where the resistance of a single plate is higher 
than the required value, two or more plates may 
be used in parallel and the total resistance is than 
inversely proportional to the number employed, 
provided that each plate is installed outside the 
resistance area of any other. This normally requi- 
res a separation of about 10 m but for sizes of plate 
generally employed, a separation of 2 m is suffi- 
cient to ensure that the total resistance will not 
exceed the value obtained from the above formula 
by more than 20 percent. Even at the latter spac- 
ing, it is generally more economical to use two 
plates in parallel, each of a given size, than one of 
twice that size. The size employed is, therefore, 
normally not greater than 1 -2 x 1 *2 m. 

Plate electrodes shall be of the size at least 60 
cm X 60 cm. Plates are generally of cast iron not 
less than 12 mm thick and preferably ribbed. The 
earth connection should bejoined to the plate at 
not less than two separate points. Plate electrodes, 
when made of GI or steel, shall be not less than 
6-3 mm in thickness. Plate electrodes of Cu shall 
be not less than 3T5 mm in thickness. 

Suitable methods of jointing are a taper pin 
driven into a reamed hole and riveted over or a 
copper stud screwed into a tapped hole and rive- 
ted. Such joints should be protected by a heavy 
coat of bitumen. The connection between the 
earth plate and the disconnecting link should be set 
vertically and the depth of setting should be such 
as to ensure that the surrounding soil is always 
damp. The minimum cover should be 600 mm ex- 
cept that where the underlying stratum is solid, 
for example, chalk or sandstone and near the sur- 
face, the top of the plate should be level with the 
top of the solid stratum. Sufficient solid stratum 
should be removed and replaced with fine soil or 
other suitable infill to ensure as low a resistance 
as possible. 

The use of coke breeze as an infill is not 
recommended as it may result in rapid corrosion 
not only of the electrode itself but also of cable 
sheaths, etc, to which it may be bonded. 



19 



IS : 3043 - 1987 



The resistance /? (i 11 Q.) ofa 1-2 m x 1-2 
plate is given approximately by tlie formula: 



R«^ 



'WTb 



For conventional sizes, the resistance is appro- 
ximately inversely proportional to the linear di^ 
mensions, not the surface area, that is a 0-9 m x 
09 m plate would have a resistance approximately 
25 percent higher than a 12 x [-2 m plate. The 
current loading capacity ofa 1 -2 m x \-2 m plate 
is of the order of I 600 A for 2 s and 1 300 A for 
3 s. 

Plate electrodes shall be buried such that its top 
edge is at a depth not less than 1 5 m from the 
surface of the ground. However, the depth at 
which plates are set should be such as to ensure 
that the surrounding soil is always damp. Where 
the underlying stratum is solid, for example chalk 
or sandstone and near the surface, the top of the 
plate should be approximately level with the top 
of the solid stratum. 

9.2.2 Pipes or Rods ^ThQ resistance of a pipe 
or rod electrode is given by: 



R^ 



100 p 
2ni 



logo ^T- O^n^S 



where 

/ = length of rod or pipe (in cm), 
d = diameter of rod or pipe in cm, and 
p = resistivity of the soil (in Q,m) 
(assumed uniform). 

The curves of Fig. 11 are calculated from this 
equation for electrodes of 13, 25 and 100 mm dia- 
meter respectively in a soil of lOOlQ.m respectively. 
Change of diameter has a relatively minor effect 
and size of pipe is generally governed by resis- 
tance to bending or splitting. It is apparent that 
the resistance diminishes rapidly with the first few 
feet of driving, but less so at depths greater than 
2 to 3 m in soil of uniform resistivity. 

A number of rods or pipes may be connected 
in parallel and the resistance is then practically 
proportional to the reciprocal of the number em- 
ployed so long as each is situated outside the resis- 
tance area of any other. In practice, this is satis- 
fied by a mutual separation equal to the driven 
depth. Little is to be gained by separation beyond 
twice the driven depth. A substantial gain is 
effected even at 2 m separation. 

Pipes may be of cast iron of not less than 100 
mm diameter, 2-5 to 3 m long and 13 mm thick. 
Such pipes cannot be driven satisfactorily and 
may, therefore, be more expensive to instal than 
plates for the same effective area. Alternatively, 
mild steel water-pipes of 38 to 50 mm diameter are 
sometimes employed. These can be driven but are 
less durable than copper rods. 



Driven rods generally consist of round copper, 
steel-cored copper or galvanized steel (see 9.2,8) 
13, 16 or 19 mm in diameter from 1 220 to 2 440 
mm in length. 



250 




itNGTH or Pipe (mi 



FIG. 11 EFFECT OF LENGTH OF PIPE ELECTRODE 

ON CALCULATED RESISTANCE FOR SOIL 

RESISTIVITY OF 100 Q m (ASSUMED UNIFORM) 

Cruciform and star shaped sections are also 
available and are more rigid while being driven, 
but the apparent additional surface does not con- 
fer a noticeable advantage in current-carrying 
capacity or reduction of resistance. In circumstan- 
ces where it is convenient to do so, the addition 
of radial strips will be advantageous. 

Such rods may be coupled together to give 
longer lengths. Except in special conditions, a 
number of rods in parallel are to be preferred to 
a single long rod. Deeply driven rods are, how- 
ver, effective where the soil resistivity decreases 
with depth or where substrata of low resistivity 
occur at depths greater than those with rods, for 
economic reasons, are normally driven. In such 
cases the decrease of resistance with depth of dri- 
ving may be very considerable as is shown by the 
measurements plotted in Fig. 12 for a number of 
sites; for curves Ai and A2, it was known 
from previously sunk boreholes that the soil 
down to a depth between 6 and 9 m consisted of 
ballast, sand and gravel below which occurred 
London clay. The rapid reduction in resistance, 
when the electrodes penetrated the latter, was 
very marked. The mean resistivity up to a depth 
of 8 m in one case was 150nm; at 1 1 m the mean 
value for the whole depth was 20 Q m moving to 
the low resistivity of the clay stratum. Similarly 
for curve C, the transition from gravely soil to 
clayey at a depth of about 1-5 m was very effec- 
tive. In the case of curve B, however, no such mar- 
ked effect occurred, although there was a gradual 



20 



IS : 3043 - 1987 



300 
200 

too 

80 
6p 

40 
! 20^ 



2 

5 10 
ce 6 



4' 




P = 20 000Q ciT. 
PrIOOOOQ cm 

•^V-Ap^SOOOQcm 



3 6 9 

LENGTH OF DRIVEN ELCCTROOE m 



FIG. 12 



CALCULATED AND EXPERIMENTAL CURVES OF RESISTANCE OF 13 mm DIA 
DRIVEN ELECTRODES 



reduction in average resistivity with increase in 
depth, as can be seen by comparison with the dot- 
ted curves, which are calculated on the assumption 
of uniform resistivity. 

Other factors that affect a decision whether to 
drive deep electrodes or to employ several rods or 
pipes in parallel are the steep rise in the energy 
required to drive them with increase in depth and 
the cost of couplings. The former can be offset by 
reducing the diameter of the rods, since a 13 mm 
diameter rod can be driven to considerable depths 
without deformation or bending if the technique 
of using a large number of comparatively light 
blows is adopted rather than a smaller number of 
blows with a sledge hammer. Power-driven ham- 
mers suitable for this purpose are available. 




9.2.3 Strip or Conductor Electrodes — These 
have special advantages where high resistivity soil 
underlies shallow surface layers of low resistivity. 
The minimum cross-sectional area of strip electro- 



des shall be according to 12.1.1. If round conduc- 
tors are used as earth electrodes, their cross- 
sectional area shall not be less than the sizes 
recommended for strip electrodes. The resistance 
R is given by: 



loop 

2ni ' 



iogc 



2/g 
w t 



ohms 



where 



p = resistivity of the soil (in H.m) (assu- 
med uniform); 

/ = length of the strip in cm; 

w = depth of burial of the electrode in cm; 
and 

t '-=■ wndth (in the case of strip) or twice 
the diameter (fir conductors) in cm. 

Care should be taken in positioning these elec- 
trodes, especially to avoid damage by agricultural 
operations. 

Figure 13 shows the variation of calculated 
earth-resistance of strip or conductor electrodes 



21 



IS : 3043 - 1987 



with length for a soil resistivity of 100 Q.m. The 
effect of conductor size and depth over the range 
normally used is very small. 



cessary in most circumstances subject to the pro- 
vision of earthing facilities that are satisfactory 
before these bonding connections are made. 



If several strip electrodes are required for con- 
nection in parallel in order to reduce the resis- 
tance, they may be installed in parallel lines or they 
may radiate from a point. In the former case, the 
resistance of two strips at a separation of 2.4 m is 
less than 65 percent of the individual resistance of 
cither of them. 



12 

to- 
e 

2 



60 120 ISO 2^0 300 
lENGlHtmJ 

FIG. 13 EFFECT OF LENGTH OF STRIP OR 

CONDUCTOR ELECTRODES IN CALCULATED 

RESISTANCE FOR SOIL RESISTIVITY OF 100 Qm 

(ASSUMED UNIFORM) 



9.2.4 Water Pipes — Water pipes shall not be 
used as consumer earth electrodes. 



NOTE — In urban districts and other areas where 
piped water supply is available the use of water pipes 
for consumers' earth electrodes has been common in the 
past. Though this was generally very effective when 
consumers' pipes and water-mains to which they were 
connected were all metal-to-metal joints, the use of 
public water-pipes for this purpose has not been accep- 
table for many years because of the use of nonconduc- 
ting material for pipes on new installations and for 
replacement purposes. Jointing techniques now being 
used do not ensure electrical continuity of metallic 
pipes. 



For new installations, therefore, a public water- 
pipe may not be used as a means of earthing. 
Metallic pipe systems of services other than water 
service (for example, for flammable liquids or 
gases, heating systems, etc) shall not be used as 
earth electrodes for protective purposes. Bonding 
of the water service with the exposed metalwork 
of the electrical installation (on the consumers' 
side of any insulating insert) and any other extra- 
neous metalwork to the installation earthing 
terminal is, however, permissible and indeed ne- 



For existing installations in which a water pipe 
is used as a sole earth electrode; an independent 
means of earthing should be provided at the first 
practicable opportunity. 

9.2.5 Cable Sheaths — Where an extensive 
underground cable system is available, the lead 
sheath and armour form a most effective earth- 
electrode. In the majority of cases, the resistance 
to earth of such a system is less than I Q.A freshly 
installed jute or hessian served cable is insulated 
from earth, but the insulation resistance of the 
jute deteriorates according to the moisture content 
and nature of the soil. However, cable sheaths arc 
more commonly used to provide a metallic path 
to the fault current returning to the neutral. 



9.2.6 Structural Steelwork — The resistance to 
earth of steel frames or reinforced concrete build- 
ings will vary considerably according to the type 
of soil and its moisture content, and the design of 
the stanchion bases. For this reason, it is essential 
to measure the resistance to earth of any structu- 
ral steetwork that it is employing and at frequent 
intervals thereafter. 



NOTE — special care is necessary where the cons- 
truction includes prestressed concrete. 

9.2.7 Reinforcement of Piles — At power stations 
and large substations, it is often possible to secure 
an effective earth-electrode by making use of the 
reinforcement in concrete piles. The earth strap 
should be bonded to a minimum of four piles and 
all the piles between the bonds should be bonded 
together. Each set of four piles should be connec- 
ted to the niai nngearthi-strap of the substation. 



9.2.8 Calhodically Protected Structures — Cathodi c 
protection is normally applied to ferrous struc- 
tures in order to counteract electrolytic corrosion 
at a metal to electrolyte interface. 



The electrolyte is generally the ground in 
which the structure is either wholly or partially 
buried and the protection system relies upon 
maintaining the metalwork at a slightly more 
negative potential than it would exhibit by half 
cell measurements, if no corrective action had 
been taken. 



The application of cathodic protection varies 
according to circumstances between bare metal in 
contact with ground and metal that has been 



22 



IS : 3043 - 1987 



deliberately coated or wrapped against corrosion. 
In the latter case, cathodic protection is used to 
supplement the coating and guard against localized 
corrosion due to coating Haws or faults. Protective 
system current drain is proportional to the area of 
bare metal in earth contact and if a normal 
earthing electrode is attached to a cathodically 
protected structure, the increased drain current 
taken by the electrode could be completely unac- 
ceptable. This is especially true where the system 
has been designed to protect a well wrapped or 
coated structure. 

Nevertheless, there may be a necessity to con- 
nect earth electrodes to cathodically protected 
structures, especially where the coating or wrapp- 
ing tends to electrically insulate the structure from 
ground, for example: 

a) diversion of earth fault currents from elec- 
trical apparatus mounted on the structure; 

b) diversion of stray current to ground, a prob- 
lem often met where well coated pipelines 
are substantially parallel to the route of a 
high voltage overhead line; 

c) prevention of elevated voltages where struc- 
tures encroach into hazardous (flamma- 
ble) areas; and 

d) Prevention of power surges into the appara- 
tus providing cathodic protection, or similar 
invasion of delicate low current instrumen- 
tation circuits. 

In addition to the guidance given in 9.3, 
selection of metals for earth electrodes and deter- 
mination of their ground contact area is most 
important where cathodically protected structures 
are involved. 

The material selected should exhibit a galvanic 
potential with respect to ground as nearly equal 
to that exhibited by the stmcture in its natural 
or unprotected condition. For ferrous structures, 
austenitic iron (austenitic cast nickel chromium 
alloy with spheroidal graphite present) is often 
used. Vertically driven rods of this material are 
preferred in order to minimize contact area and 
thus reduce cathodic protection drain, whilst 
obtaining optimum performance fi*om the electro- 
de. Copper should be avoided, wherever possible, 
not only for its increased drain but also for its 
ability to become cathodic to the protected struc- 
ture. Magnesium or zinc electrodes have been 
used successftilly, but are anodic to the protected 
structure and thus sacrificial in action. 

9.3 Selection of Metals for Earth-Electrodes 

— Although electrode material does not affect ini- 
tial earth resistance, care should be taken to select 
a material that is resistant to corrosion in the type 
of soil in which it will be used. Tests in a wide 
variety of soils have shown that copper, whether 



tinned or not, is entirely satisfactory (subject to 
the precautions given in this subclause), the aver- 
age loss in weight of specimens 150 mm x 25 
mm X 3 mm buried for 12 years in no case exceed 
0-2 percent per year. Coiresponding average 
losses for unprotected ferrous specimens (for 
example, cast iron, wrought iron or mild steel) used 
in the tests were as high as 2-2 percent per year. 
Considerable and apparently permanent protection 
appears to be given to mild steel by galvanizing, 
the test showing galvanized mild steel to be little 
inferior to copper with an average loss not greater 
than 0*5 percent per year. Only in a few cases 
was there any indication in all these tests that 
corrosion was accelerating and in these cases the 
indications were not very significant. 

The possibility on damage to cables and other 
underground services and structural metalwork in 
the vicinity of earth-electrode due to electrolytic 
action between dissimilar materials should not be 
overlooked when the material for earth-electrodes 
is selected. Materials compatible with other metal 
structures in the vicinity should be selected or 
other remedial action taken. 



It may be essential to use materials of types 
other than those mentioned earlier in special cir- 
cumstances, when cathodically protected structures 
such as pipelines are encountered. 



A modem high pressure gas pipeline, wrapped 
and cathodically protected may have a galvanic 
potential of — 0-5 V, the accepted material of cop- 
per for an earth electrode with a galvanic potential 
of — 0-2 V decreases the total galvanic voltage and 
increases the need for current from the corrosion 
protection impressed current system, when the 
earth electrode is connected to the pipeline. 

An earth electrode with a galvanic potential 
nearer to the protected structure has to be used to 
overcome the above and be certain the pipeline is 
being protected. Such a material is termed an 
austenitic iron and is an austenitic cast nickel- 
chromium alloy, with spheroidal graphite present. 

It may be necessary to earth the pipeline for 
one or more of the following reasons; 

a) It should not on its own be a carrier of any 
low voltage fault current, 

b) It may have low voltage equipment connec- 
ted to it, for example, for the purpose of 
valve operation; 

c) It may have instrumentation connected to 
it that require it to be earthed for this pur- 
pose and to provide a signal reference earth 
as well as for earthing requirement relative 
to electrical equipment used in hazardous 
areas; and 



23 



IS : 3043 - 1987 



d) It may require connection to earth at 
points to discharge unwanted induced cur- 
rents and vohages fi:om other sources such 
as overhead hues. 

These four points lead to a compromise 
between the need to have a low earth value for in- 
strumentation reference purposes, which may 
require a lot of buried metal, and a reasonable 
earth value for electrical purposes against the corro-^ 
sion protection requirement to have a minimum of 



buried bare metal connected to the pipeline, and 
thus drawing a corrosion protection current that 
may be required by the pipehne. 

9.4 Typical installations of pipe earth electrode 
and plate earth electrode are shown in Fig. 14 
and 15. 

9.5 Typical Method for Jointing of Conductors -^ 
Methods of jointing conductors are shown in 
Fig. 16. 



CONOUIT 
£M8ED0ED> 



Jli 



40Q 



-..+014 



^r'^^;' WIRE MESH :^:y: fZSl^^ f; 



C I COVER HI NGCO 
'to t\ f«AM^ 



^7?s7p; ^v/ A\ ] ; 






o 

-11 




NOTE — After laying the earth from the earth bus to the electrode through the PVG conduits at the pit 
entry conduits should be sealed with bitumin compound. 

All dimensions in millimetres. 
FIG. 14 TYPICAL ARRANGEMENT OF PIPE ELECTRODE 



24 



IS : 3043 - 1987 



CAS? i^ore 
J CI C( 




^ 10 Of BOtr LENGTH 
50fnm a 100 m (AFTER 
FlxrNG»THE OUTSIDE 
SURFACE SHOULD iE 
COVERED WtTM 0ITUMIN) 



\X Gl PLATE FOR CLAMP 



DETAIL C 



All dimensions in millimetres. 
FIG. 15 TYPICAL ARRANGEMENT OF PLATE ELECTRODE 



PUSH BUTTON 
STAnON/STARTER' 




16A Earthing Arrangement for Motors with Push Button Station/Starter Earth Connections to 
Starter Looped from Earth Connections of Motor 

FIG 16. TYPICAL EARTHING CONNECTION DETAILS — Contd 



25 



IS: 3043 - 1987 




TINNER 

COPPER 

CABLE 



^.^r-Gl BOLTS AND WUTS^rLAT 
^.^<7 WASHER AND Qt SPRING 
^^^-^ WASHER UFTER FIXiNG^THE 
OUT SIDE SURFACE SHOULD 
8£ COVERED WITH StTUMIH 
OR GREASE) 



EaUIPMENT/STRIP 



SOCKET 



16B Arrangement of Double Earth Connection to Equipments (Strip to Conductor Connection) 



BRAZING FOR COPPER AND WELDING 
FOR ALUMINIUM 




OVERLAP MIN. 50 mm ^^cOPPbT ' 



16C Straight Joint (Strips) 



16D T Joint (Strips) 



FLAT WASHER AND SPRING V^ASHERCGI) 
TO BE CONNECTED USING GI BOLTS 

AND NUTS-- 



EQUIPMENT/STRIP 
( Ai/Cu J 





:.2mm TfNNEO COPPER 
BINDING WIRE AND THE 
JOJNT TO BE SOLDERED 



■ ALUMINIUM/COPPER 
CONDUCTOR 



ALUMfNiUM 
OR COPPER 



16E Arrangement of Strip to Strip and Strip 
to Equipment Connection 



16F Conductor to Conductor Joint 
(Round Conductors) 



FIG. 16 TYPICAL EARTHING CONNECTION DETAILS 



26 



IS : 3043 - 1987 



10. CURRENT DENSITY AT THE SURFACE 
OF AN EARTH-ELECTRODE 

10.1 An earth electrode should be designed to 
have a loading capacity adequate for the system 
of which it forms a part, that is, it should be capa- 
ble of dissipating without failure the energy in the 
earth path at the point at which it is installed 
under any condition of operation on the system. 
Failure is fundamentally due to excessive temper- 
ature rise at the surface of the electrode and is 
thus a function of current density and duration as 
well as electrical and thermal properties of the 
soil. 

In general, soils have a negative temperature 
coefficient of resistance so that sustained current 
loading results in an initial decrease in electrode 
resistance and a consequent rise in the earth fault 
current for a given applied voltage. As soil mois- 
ture is driven away from the soil-electrode inter- 
face, however, the resistance increases and will 
ultimately become infinite if the temperature-rise 
is sufficient. 

10.2 Three conditions of operation require 
consideration, that is, long-duration loading as 
with normal system operation; short-time over- 
loading as under fault conditions in directly ear- 
thed systems, and long-time overloading as under 
i^ault conditions in systems protected by arc- 
suppression coils. 

10.3 The little experimental work which has been 
done on this subject by experts at the inter- 
national level has been confined to model tests 
with spherical electrodes in clay or loam of low 
resistivity and has led to the following conclusions: 

a) Long-duration loading due to normal un- 
balance of the system will not cause failure 
of earth-electrodes provided that the cur- 
rent density at the electrode surface does 
not exceed 40A/m'. Limitation to values 
below this would generally be imposed by 
the necessity to secure a low-resistance 
earth, 

b) Time to failure on short-time overload is 
inversely proportional to the specific loa- 
ding, which is given by /^, where / is the 
current density at the electrode surface. 
For the soils investigated, the maximum 
permissible current density, / is given by 



11. VOLTAGE GRADIENT 

EARTH ELECTRODES 



AROUND 



7*57 X 10« 



Ajm^ 



where 



/ = duration of the earth fault (ins); 
p ^ resistivity of the soil (in Q.m). 

Experience indicates that this formula is 
appropriate for plate electrodes. 



11.1 Under fault coditions, the earth electrode is 
raised to a potential with respect to the general 
mass of the earth that can be calculated from the 
prospective fault current and the earth resistance 
of the electrode. This results in the existence of 
voltages in the soil around the electrode that may 
be injurious to telephone and pilot cables, whose 
cores are substantially at earth potentional, owing 
to the voltage to which the sheaths of such cables 
are raised; the voltage gradient at the surface of 
the ground may also constitute a danger to life, 
especially where cattle are concerned. The former 
risk arises mainly inconnection with large elect- 
rode systems as at power stations and substations. 

11.2 Danger to animals occurs principally with 
pole-mounted substations on low-voltage systems. 
In rural areas, it is by no means uncommon for 
the earth-path resistance to be such that faults are 
not cleared within a short period and in such 
cases, animals, which frequently congregate near 
a pole, are liable to receive a dangerous shock. 
The same trouble sometimes occurs at famis where 
earth electrodes are provided for individual app- 
liances. An effective remedy is to earth the neutral 
conductor at some point on the system inaccessi- 
ble to animals rather than earthing the neutral at 
the transformer itself Alternatively, an effective 
method is for pipe or rod electrodes to be buried 
with their tops below the surface of the soil and 
connection made to them by means of insulated 
leads. The maximum voltage gradient over a span 
of 2 m adjacent to a 25 mm diameter pipe elec- 
trode is reduced from 85 percent of the total elec- 
trode potential when the top of the electrode is at 
ground level to 20 and 5 percent when it is buried 
3 and 1*0 m respectively. 

11.3 Earth electrodes, other than those used for 
the earthing of the fence itself, should not be in- 
stalled in proximity to a metal fence, to avoid the 
possibility of the fence becoming live and thus 
dangerous at points remote from the substation or 
alternatively giving rise to danger within the resis- 
tance area of the electrode by introducing a good 
connection with the general mass of the earth. 

12. CONNECTIONS TO EARTH ELEC- 
TRODES - EARTHING AND PROTEC- 
TIVE CONDUCTORS 

12.0 General 

12.0.1 The materials used for making con- 
nections have to be compatible with the earth rod 
and the copper earthing conductor so that galvanic 
corrosion is minimized. In all cases, the connec- 
tions have to be mechanically strong. 

12.0,2 For large earthing installations, such as 
at major substations, it is common to make provi- 
sion for the testing of earth electrodes. This is 



27 



IS : 3043 - 1987 



achieved by connecting a group of rod driven 
electrodes to the main earth grid through a bolted 
link adjacent to the electrodes in a sunken con- 
crete box. Simpler disconnecting arrangements 
(or none at all) may be acceptable for small 
earthing installations. 

12.1 Earthing Conductors 

12.1.1 Earthing conductors shall comply 
with 12.2.2 and, where buried in the soil, their 
cross-sectional area shall be in accordance with 
Table 4. 

TABLE 4 MINIMUM CR0SS^ECT10N.\L AREA 
OF EARTHING CONDUCTORS 



Protected 
against 



Not protec- 
ted against 
corrosion 



MFXHANICALLY 
PROTECTED 

According to 12.2.2 
with a minimum of 
16 mm" (Cu) or (Fe) 



MECHANSCALLY 
UNPROTECTED 

16 mmt,(Cu 
16 mm" (Fe 



25 mm" (Cu) 
50 mm^ (Fe) 



12.1.2 The connection of an earthing conduc- 
tor to an earth electrode shall be soundly made 
and electrically satisfactory. Where a clamp is 
used, it shall not damage the electrode (for exam- 
ple, a pipe) or the earthing conductor. 

12.1.3 Main Earthing Terminals or Bars — In 
every installation, a main earthing terminal or bar 
shall be provided and the following conductors 
shall be connected to it: 

a) earthing conductors; 

b) protective conductors; and 

c) functional earthing conductors, if required. 

Means shall be provided in an accessible posi- 
tion for disconnecting the earthing conductor. 
Such means may conveniently be combined with 
the earthing terminal or bar to permit measure- 
ment of the resistance of the earthing arrange- 
ments This joint shall be disconnectable only by 
means of a tool, mechanically strong and ensure 
the maintenance of electrical continuity. 

12.1 Protective Conductors 

12.2.1 Types ofProtective Conductors 

12.2.1.1 Protective conductors may com- 
prise: 

a) conductors in multicore cables; 

b) insulated or bare conductors in a common 
enclosure with live conductors; 

c) fixed bare of insulated conductors; 

d) metal coverings, for example, the sheaths, 
screens and armouring of certain cables 
(further requirements under consideration) 
{see Note 1 ) ; 

e) metal conduits or other metal enclosures 
for conductors (further requirements under 
consideration) {see Note 2 ) ; and 



f) certain extraneous conductive parts. 

NOTE 1 — Where the metal sheaths of cables are 
used as earth continuity conductors, every joint in 
such sheaths shall be so made that its current 
carrying capacity is not less than that of the sheath 
itself. Where necessary, they shall be protected 
against corrosion. Where non-metallic joint-boxes 
are used, means shall be provided to maintain the 
continuity such as a metal strip having a resistance 
not greater than that ofthe sneath of the largest 
cable entering the box. 

NOTE 2 — Metal conduit pipe should generally 
not be used as an earth-continuity conductor but 
where used, a very high standard of workmanship 
in installation is essential. Joints shall be so made 
that their current carrying capacity is not loss 
than that of the conduit itself. Slackness in joints 
may result in deterioration and even complete loss 
of continuity. Plain slip or pin-grip sockets are 
insufficient to ensure satisfactory continuity of 
joints. In the case of screwed conduit, lock nuts 
should also be used. 

12.2.1.2 The metallic covering including 
sheaths (bare or insulated) of certain wiring, in 
particular the sheaths of mineral-insulated cables, 
and certain metallic conduits and trunking for 
electrical purposes (types under consideration) 
may be used as a protective conductor for the 
corresponding circuits, if their electrical continuity 
can be achieved in such a manner ensuring pro- 
tection against deterioration and they permit 
connection of other protective conductors at pre- 
determined tap oif points. Other conduits for 
electrical purposes shall not be used as a protective 
conductor. 

12.2. L3 Extraneous conductive parts may be 
used as a protective conductor if they satisfy the 
following four requirements: 

a) their electrical continuity shall be assured 
either by construction or by suitable con- 
nections in such away as to be protective 
against mechanical, chemical or electro- 
chemical deterioration; 

b) their conductance shall be at least equal to 
that resulting from the application 
of 12.2.2; 

c) unless compensatory measures are provided 
precautions shall be taken against their 
removal; and 

d) they have been considered for such a use 
and, if necessary, suitably adapted. 

The use of metallic water pipes is permitted, 
provided the consent of a person or body respon- 
sible for the water system is obtained. Gas pipes 
shall not be used as protective conductors. 

12.2.1.4 Extraneous conductive parts shall 
not be used as PEN conductors. 

12.2.2 Minimum Cross-Sectional Area 

12.2.2.0 The cross-sectional area of protec- 
tive conductors shall either be: 

a) calculated in accordance with XIJIJIA^ or 

b) selected in accordance with 12.2.2.2. 



28 



IS : 3043 - 1987 



In both cases, 12.2.2.3 shall be taken into 
account. 

NOTE — The installation should be so prepared 
that equipment terminals are capable ofaccepting these 
protective conductors. 

12,2.2.1 The cross-sectional area shall be so 
calculated that the current density value deter- 
mined by the following formula is not exceeded 
(applicable only for disconnection times not 
exceeding 5 s). 



where 
S 



■ cross-sectional area, in square milli- 
metres; 

value (ac, rms) of fault current for 
a fault of negligible-impedance, which 
can flow through the protective 
device, in amperes; 

operating time of the disconnecting 
device, in seconds; and 

NOTE — Account should be taken of the 
current-limiting effect of the circuit impe- 
dances and the limiting capability (joule 
integral) of the protective device. 

= factor dependent on the material of 
the protective conductor, the insula- 
tion and other parts, and the initial 
and final temperatures. Values of/: 
for protective conductors in various 
use or service for t = I and 3 s res- 
pectively are given in Table 6A to 
6D. 



130 



The k factors for protective conductors of 
copper, steel and aluminium are shown in Fig. 17 
to 19. 

If application of the formula produces non- 
standard sizes, conductors of the nearest higher 
standard cross-sectional area shall be used. 

NOTE I — It is necessary that the cross-sectional 
area so calculated be compatible with the conditions 
imposed by fault loop impedance. 

NOTE 2 — Maximum permissible temperatures for 
joints should be taken into account, 

NOTE 3 — Values for mineral-insulated cables are 
under consideration. 

Method of deriving the factor k 

The factor k is determined from the formula: 



k 

where 



-v^ 



c ( ^ -f 20 ) 



i^-'n^) 



Qc = volumetric heat capacity of conduc- 
tor material (j/°Cmm"), 
B = reciprocal of temperature coefficient 
of resistivity at 0°C for the conduc- 
tor ("C), 

doi) = electrical resistivity of conductor 
material at 20°C (i^-mm), 

S] = initial temperature of conductor 

("O, and 

Qt = final temperature of conductor (°C). 

These material constants are given in Table 5. 




100 



200 300 

FINAl TeMPil«ATUR£ ""C 



iOO 



$00 



FIG. 17 k FACTORS FOR COPPER PROTECTIVE CONDUCTORS {See 12.2.2.1) 

29 




100 200 3d0 

FINAL TEMPERATURE *C — 



FIG. 



k FACTORS FOR STEEL PROTECTIVE CONDUCTORS 



150 



100 



SO 





^ 


"^^^""""^ 




^ 


INITIAL 
TEMPERATURE 

30'C 
40* 




^ 






70' 
90"C 


7 









100 



400 



SOO 





FIG. 19 


PIWAt T£MPERAI 

k FACTORS FOR ALU MINIUM 


PROTECTIVE CONDUCTORS 




MATERIAL 

Copper 
Aluminium 
Lead 
Steel 


i5(°C) 

234-5 
228 
230 
202 


TABLE 5 MATERIAL CONSTANTS 

Qc(J/^Cmm^) §20 (a mm) 

3-45 X 10"^ 17-241 X 10 "^ 
2-5 X lO"-' 28 264 x lO'*^ 
1-45 X 10"^ 214 X 10-^ 
3-8 X 10"' 138 X 10"^^ 




3iio 

226 
148 

42 
78 



30 



IS.: 3043 -1987 



TABLE 6 Ct'RRENT RATING OF VARIOUS PROTECTIVE EARTHING MATERIALS 

(Clauses n22 ami 19.2) 



6A Bare Conductor with No Risk of Fire or Danger to Any Other Touching or Surrounding Material 

Boiniciaty Conditiotis : Initial Temperature. 40'^C Final temperature 395''C for copper; 325^^0 for aluminium; 500°C 
for steel 



MATERIAL COPPER 

I s current rating in y\/mni" (ki) 205 

3 s current rating in A/mm^ (A'3) 118 



ALUMINIUM 
!26 
73 



STEEL 

80 
46 



6B insulated Protective Conductors not Incorporated in Cables or Bare Conductors Touching Other 

Insulated Cables 

Boundcwv Conditions : Initial Temperature : 40°C. Final temperature: 160°C for PVC, 220°C for butyl rubber 
250°C for XLPE/EPR 

MATERIAL COPPER ALUMINIUM STEEL 



INSULA 1 lUIN 


'pviT 


Butyl Rubber 


xlpeT ' 

EPR 


"""pvc" 


B 


Lityl 


Rubber 


XLPE? 
EPR 


T^VC 


Butyl 
Rubber 


XLPE/ 
EPR 


1 s current rating 


136 


160 


170 


90 






106 


112 


49 


5<S 


62 


in A/ mm" [k]) 
























3 s current rating 


79 


92 


98 


52 






61 


65 


28 


33 


36 


in A/ mm' {k:,) 

























6C Protective Conductor as a Core in Multicore Cables 



BoLindaty Conditions: 

PVC 

Butyl Rubber 

XLPE/EPR 



MATERIAL 

INSULATION 



I s current rating 
in A/mm" (A'l) 

3 s current rating 
in A/mm" (A-3) 



66 



Initial Temperature 

arc 

85°C 
90°C 

COPPER 



PVC Butyl Rubber XLPE/EPR 

115 134 143 



77 



83 





Final Temperature 




160°C 






220°C 






250^C 






ALUMINIUM 




PVC 

76 


Butyl Rubber 

89 


XLPE/EPR 
94 


44 


51 


54 



6D Protective Bare Conductors in Hazardous Areas Where There is Risk of Fire from Petroleur 

Bound Oil or Other Surrounding Material 



Boundary' Conditions : Initial Temperature : 40°C; Final Temperature 150'^C/200°C. 
MATERIAL COPPER ALUMINIUM 

1 s current rating in A/mm" (i|) 131/153 86/101 

3 s current rating in A/mm" (A-;,) 76/88 50/58 



STEEL 

47/56 
27/32 



12,2.2.2 The cross-sectional area of the pro- 
tective conductor shall be not less than the appro- 
priate value shown in Table 7. In this case, 
checking of compliance with 12.2.2.1 is usually not 
necessary. 

ff the application of this table produces non- 
standard sizes, conductors having the nearest 
higher standard cross-sectianal area are to be 
used. 



TABLE 7 CROSS SECTION OF PROTECTIVE 
CONDUCTOR 

CROSS-SECTIONAL AREA OF MINIMUM CROSS-S fiCTION A L 



PHASE CONDUCTORS OF 

THE INSTALLATION 

S (mnr) 

S < 16 
16 < 5 < 35 

S > 35 



AREA OF THE CORRES- 
PONDING PROTECTIVE 
CONDUCTOR 
Sp (mnr) 

16 
See 12.2.2.1 



31 



IS : 3043 - 1987 



The values in Table 7 are valid only if the 
protective conductor is made of the same metal as 
the phase conductors. If this is not so, the cross"' 
sectional area of the protective conductor is to be 
determined in a manner which produces a con- 
ductance equivalent to that which results from the 
application of Table 7 (see also 18.3.3). 

12.2.2.3 The cross-sectional area of every 
protective conductor which does not form part of 
the supply cable or cable enclosure shall be, in 
any case, not less than: 

a) 2-5 mm^, if mechanical protection is provi- 
ded; and 

b) 4 mm", if mechanical protection is not 
provided. 

12.2.3 Preservation of Electrical Continuity of 
Protective Conductors 

12.2.3.1 Protective conductors shall be suit- 
ably protected against mechanical and chemical 
deterioration and electrodynamic forces. 

12.2.3.2 Joints of protective conductors 
shall be accessible for inspection and testing 
except in compound-filled or encapsulated joints. 

12.2.3.3 No switching device shall be inser- 
ted in the protective conductor, but joints which 
can be disconnected for test purposes by use of 
a tool may be provided. 

12.2.3.4 Where electrical monitoring of 
earth-continuity is used, the operating coils shall 
not be inserted in protective conductors. 

12.2.3.5 Exposed conductive parts of appa- 
ratus shall not be used to form part of the protec- 
tive conductor for other equipment except as 
allowed by the preconditions in 12.2.1.2, 



13. EARTHING ARRANGEMENTS 
PROTECTIVE PURPOSES 



FOR 



NOTE — For protective measures for various systems 
of earthing, see Section 3. 

13.1 Protective Conductors used with Over- 
current Protective Devices 

13.1.1 When overcurrent protective devices 
are used for protection against electric shock, the 
incorporation of the protective conductor in the 
same wiring system as the live conductors or in 
their immediate proximity is strongly recommen- 
ded. 

13.2 Earthing and Protective Conductors for 
Fault-Voltage-Operated Protective Devices 

13.2.1 An auxiliary earth electrode shall be 
provided electrically independent of all other 
earthed metal, for example, constructional metal- 
work, pipers, or metal-sheathed cables. This 
requirement is considered to be fulfilled if the 



auxiliary earth electrode is installed at a specified 
distance from all other earthed metal (value of 
distance under consideration). 

13.2.2 The earthing conductor leading to the 
auxiliary earth electrode shall be insulated to 
avoid contact with the protective conductor or 
any of the parts connected thereto or extraneous 
conductive parts which are, or may be, in contact 
with them. 

NOTE — This requirement is necessary to prevent 
the voltage-sensitive element being, inadvertently 
bridged. 

13.2.3 The protective conductor shall be con- 
nected only to the exposed conductive parts of 
those items of electrical equipment whose supply 
will be interrupted in the event of the protective 
device operating under fault conditions. 

13.2.4 Excessive Earthed-leakage Current — Under 
consideration. 



14. EARTHING ARRANGEMENTS 
FUNCTIONAL PURPOSES 



FOR 



14.1 General — Earthing arrangements for func- 
tional purposes shall be provided to ensure correct 
operation of equipment or to permit reliable and 
proper functioning of installations. 

(Further requirements under consideration). 

14.2 Low Noise —^ee 39.22. 

15. EARTHING ARRANGEMENTS FOR 
COMBINED PROTECTIVE AND 

FUNCTIONAL PURPOSES 

15.1 General — Where earthing for combined 
protective and functional purposes is required, 
the requirements for protective measures shall 
prevail. 

15.2 PEN Conductors 

15.2.1 In TN systems, for cables in fixed insta- 
llations having a cross-sectiona! area not less than 
10 nW for copper and 16 mm" for aluminium, a 

single conductor may serve both as protective 
conductor and neutral conductor, provided that 
the part of the installation concerned is not pro- 
tected by a residual current-operated device. 

However, the minimum cross-sectional area 
of a PEN conductor may be 4 mm\ provided that 
the cable is of a concentric type conforming to 
Indian Standards and that duplicate continuity 
connections exist at all joints and terminations in 
the run ofthe concentric conductors. 

15.2.2 The PEN conductor shall be insulated 
for the highest voltage to which it may be subjec- 
ted to avoid stray currents. 

NOTE — The PEN conductor need not be insulated 
inside switchgear and controlgear assemblies. 



32 



IS : 3043 - 1987 



15.2.3 If from any point of the installation the 
neutral and protective functions arc provided by 
separate conductors, it is inadmissible to connect 
these conductors to each other from that point. 
At the point of separation, separate terminals or 
bars shall be provided for the protective and 
neutral conductors. The PEN conductor shall be 
connected to the terminal or bare intended for 
the protective conductor. 

16. EQUIPOTENTIAL BONDING 
CONDUCTORS 

16.1 Minimum Cross-Sectional Areas 



16.1.1 Equipotential Bonding Conductors — 
Seen,l,lA. 

16.1.2 Bonding of Water Meters — Bonding of 
water meters is not permitted {see 9.2.4). 

16.2 Non-Earthed Equipotential Bonding — 

Under consideration. 

17. TYPICAL SCHEMATIC OF EARTHING 
AND PROTECTIVE CONDUCTORS 

17.1 A typical schematic of earthing and protec- 
tive conductors is given in Fig. 20. 



EE 



^SZ 






£ 



2ZZZ2Z2ZZZZZZZZZZZZn 






INS TAIL AT ION 
EARTHING 



^ 



CEJ: 



X^777//////////////////,^ZZ22 






E£ 






EQUIPMENT 
EA^THfNa 



M = Exposed conductive parts 
P = Incoming metaliic service 
C = Extraneous conductive parts 
EE = Earth electrode 
1 = Equipotential bonding conductor (in case of small domestic installations 1 takes the form 
of neutral link) 

2 = Protective conductor (in duplicate) 

3 = Earthing conductor 

FIG. 20 EARTHING ARRANGEMENTS AND PROTECTIVE CONDUCTORS 



33 



IS : 3043 - 1987 

SECTION 3 EARTH FAULT PROTECTION IN CONSUMER'S PREMISES 



18. EARTH FAULT 
INSTALLATIONS 



PROTECTION 



IN 



18.0 Basic Philosophy of Earth Fault 
Protection 

18.0.1 The rules given in this Section are 
applicable to installation below 1 000 V ac. 

18.0,2 Amongst other things, protection against 
shock in case of a fault (protection against 
indirect contact) is provided by automatic 
disconnection of supply. This protective measure 
necessitates coordination of the types of system 
earthing and the characteristics of the protective 
devices. This Section discusses the basic criteria 
for achieving this protection. 

18.0.3 Protection against electric shock both 
in normal service (protection against direct 
contact) and in case of fault (protection against 
indirect contact) can be achieved by several 
measures. Details of achieving protection through 
the choice of an appropriate protective measure 
is the subject of IS : 732*. One of such measures 
is protection by automatic disconnection of 
supply. Automatic disconnection is intended to 
prevent a touch voltage persisting for such time 
that a danger could arise. This method necessi- 
tates co-ordination of (a) the type of system 
earthing, and (b) characteristics of protective 
devices. Description of the types of system earth- 
ing permitted and the requirements for earthing 
arrangements and protective conductors vis-a vis 
protection against shock is the subject of this 
code. 

18.0.4 Protective measure by automatic dis- 
connection of supply following an insulation fault 
relies on the association of two conditions given 
below; 

a) The existence of a conducting path (fait 
loop) to provide for circulation of fault 
current (this depends on type of system 
earthing); and 

b) The disconnection of this current by an 
appropriate device in a given time. 

The determination of this time depends on 
various parameters, such as probability of fault, 
probability of a person touching the equipment 
during the fault and the touch voltage to which a 
person might thereby be subjected. 

Limits of touch voltage are based on studies 
on the effects of current on human body (see 
IS : 8437-l977t). 

18.0.5 The study of the electrical impedance 
of the human body as a function of touch voltage 
and magnitude of current flow in the body as a 



function of its duration likely to produce a given 
effect are two components which help in establi- 
shing a relationship between prospective touch 
voltage and its duration which will not result in 
harmful physiological effects for any person. 

Table 8 shows the values of disconnecting 
times / for given touch voltages for two most 
common conditions. 



TABLE 8 DISCONNECTING TIMES FOR 
DIFFERENT TOUCH VOLTAGES 



PROSPEC- 


CONDITION 


f 1* 


CONDITION 


2t 


TIVE ^ 




^ 




r o u c H 
VOLTAGE 


z, 


/ 


t 


%' 


/ 


t ' 


(V) 

25 


(Q) 


(mA) 


(s) 


(H) 

075 


(mA) 

23 


(s) 
5 


50 


1 725 


29 


5 


925 


54 


0-47 


75 


1 625 


46 


0-60 


825 


91 


0-30 


90 


1 600 


56 


0-45 


780 


115 


0-25 


110 


1 535 


72 


0-36 


730 


151 


0-18 


150 


1475 


102 


0-27 


660 


227 


010 


220 


1 375 


160 


017 


575 


383 


0035 


280 


1 370 


204 


012 


570 


491 


020 


350 


1 365 


256 


008 


565 


620 


— 


500 


1 360 


368 


0'04 


560 


893 


— 


*Dry or moist locations, 

resistance. 22 


dry skin 


and 


significant floor 


jWet locations, wet skin and low floor resistance 





*Code of practice for wiring installations. 
jGuide on effects of currents passing through the human 
body. 



18.0.6 It is necessary, therefore, to apply these 
results emanating out of IS : 8437-1977* to the 
various earthing systems. The disconnecting times 
specified for different circuits in this code follows 
basically the summary in Table 8, in addition 
taking into account the likelihood of faults and 
likelihood of contact. 

18.0.7 TN Systems — All exposed conductive 
parts shall be connected to the earthed point of 
the lower system by protective conductors. The 
protective conductors shall be earthed near each 
power transformer or generator of the installation. 
If other effective earth connections exist, it is 
recommended that the protective conductors also 
be connected to such points, wherever possible. 
Earthing at additional points as evenly as possible 
is desirable. It is also recommended that protec- 
tive conductors should be earthed where they 
enter any buildings or premises. 

The characteristics of the protective devices 
and the cross-sectional area of conductors shall be 
so chosen that if a fault of negligible impedance 
occurs any where between a phase conductor and 



*Guide on effects of currents passing through the 
human body. 



34 



IS : 3043 - 1987 



a protective conductor or exposed conductive 
part, automatic disconnection of the supply will 
occur within the minimum possible safe time. 
The time of operation would depend on the 
magnitude of the contact potential. As a general 
rule, 65 V may be cleared within 10 seconds 
and voltages of the order of 240 V and above 
shall be cleared instantaneously. 

This requirement is met if; 

where 

Zs ^ fault loop impedance, 

U = current ensuring the automatic 
operation of disconnecting device, 
and 

^0 = conventional voltage limits. 

NOTE I — Zs may be calculated or measured. 

NOTE 2 — The duration of /a permitted depends 
CD the prospective touch voltage. The touch voltage is 
calculated from the voltage of the system and the ratio 
of the impedance of the source and the fault loop. 
Higher touch voltages should be cleared in shorter 
times. 

If this condition cannot be fulfilled, supple- 
mentary bonding in accordance with 18.0.10 may 
be necessary. 

18,0.8 TT Systems — All exposed conductive 
parts collectively protected by the same protective 
device shall be interconnected by protective con- 
ductors with an earth electrode common to all 
those parts. Where several protective devices are 
used in series, this requirement applies separately 
to all the exposed conductive parts protected by 
each device. For compliance with the require- 
ment of 18.0.7 (para 2), the following shall be 
fulfilled: 



RaX u< U^ 



where 



resistance of the earthed system for 
exposed conductive parts, 

/a = operating currents of the disconne- 
cting series device or settings of 
shunt relays, and 

Uq = conventional voltage limit (3 2 V 
in case of relays with time lag). 

18.0.9 IT Systems — The impedance of the 
power system earth shall be such that on the 
occurrence of a single fault to exposed conductive 
parts or to earth, the fault current is of low value. 
Disconnection of the supply is not essential on 
the occurrence of the first fault. Protective 
measures must, however, prevent danger on the 
occurrence of two simultaneous faults involving 
different live conductors. 

The following condition shall be fulfilled: 

;?* X /fl < t/o 



where 

7?A ^ resistance of the earthed system for 
exposed conductive parts, 

/a = operating currents of the disconne- 
cting series device, and 

Uq = conventional voltage limit. 

18.0.10 Equipotential Bonding — If the condi- 
tions specified in 18.0.7 to 18.0.9 cannot be flilfi- 
Iled for automatic disconnection of supply, it is 
necessary to provide local equipotential bonding 
{see also 1^3 A). This applies to entire installa- 
tion or a part thereof, an item of apparatus or a 
location. The protective conductors for local 
bonding shall also conform to 12.2. Where doubt 
exists regarding effectiveness of supplementary 
equipotential bonding, it shall be conformed if: 



z<. 



u 



where 

Z = impedance between simultaneously 
accessible exposed conductive parts 
and extraneous conductive parts, 
and earthing system; 

4= operating current of the disconnect- 
ing series device; and 
U = conventional voltage limit. 

18.1 Basic Purpose of Earth Fault Protec- 
tion — The occurrence of an earth fault in an 
installation creates two possible hazards. Firstly, 
voltages appear between exposed conductive parts 
and extraneous conductive parts, and if these 
parts are simultaneously accessible, these voltages 
constitute a shock hazard, this condition being 
known as indirect contact. 

Secondly, the fault current that flows in the 
phase and protective conductors of the circuit 
feeding the faulty equipment (the earth fault 
may, of course, occur in the fixed wiring of the 
circuit itself) may be of such a magnitude as to 
cause an excessive temperature rise in those con- 
ductors, thereby creating a fire hazard. 

The protective measure known as 'earthed 
equipotential bonding and automatic disconnec- 
tion of the supply' is intended to give a high 
degree of protection against both hazards. The 
choice of protective device used to give disconnec- 
tion is influenced by the type of system of which 
the installation is part, because either: 

a) the earth fault loop impedance has to be 
low enough to allow adequate earth fault 
current to low to cause an overcurrent 
protective device (for example, a fuse or 
circuit breaker) in the faulty circuit to 
operate in a sufficiendy short time; or 

b) where it is not possible to achieve a low 
enough earth fault loop impedance, disco- 
nnection may be initiated by fitting either 



35 



IS : 3043 - 1987 



a residual current device or a voltage 
operated earth leakage circuit breaker 
with the former being preferred. 

18.2 Earthing of Installations 

18.2.1 Protection Against Indirect Contact (Against 
Electric Shock in Case of a Fault) — Protection 
against indirect contact is achieved by the adop- 
tion of one of the following protective measures: 

a) Safety extra low voltage; 

b) The use of Glass 11 equipment or by equi- 
valent insulation; 

c) A non-conducting location; 

d) Earth free local equipotential bonding; 

e) Electrical separation; and 

f) Earthed equipotential bonding and auto- 
matic disconnection of the supply. 

NOTE 1 — The primary concern of this Code is (d) 
and (0 while other methods of protection against indi- 
rect contact are covered in other relevant Indian 
Standard Codes of Practice. 

NOTE 2 — Item (a) requires that the nominal 
voltage of the circuit concerned does not exceed extra 
low voltage that the source has a high degree of isola- 
tion from higher voltage circuits (for example, a Class 
H safety isolation transformer) and that live parts also 
have a similar degree of isolation or separation from 
those circuits. The most important requirement, 
however, is that live parts and exposed conductive parts 
of a safety extra low voltage circuit should not be 
connected to earth, protective conductors or exposed 
conductive parts of another circuit. Where these 
general requirements are not met but the nominal 
voltage still does not exceed extra low voltage, the 
circuit is described as a functional extra low voltage 
circuit and one part of it may be connected to earth. 

NOTE 3 — Item (b) is generally applicable and 
covers the selection and use of equipment complying 
with either insulation encased Class II equipment 
('all-insulated') or metal cased Class II equipment, 
hi some cases, such as factory built assemblies of 
switchgear and controlgear, the equivalent term used 
is 'total insulation'. Item (b) can also be achieved by 
the application of suitable supplementary or reinforced 
insulation to equipment on site. 

Earthing of the equipment is not required; in fact, 
by definition there will be no facility for earthing 
provided in Class II equipment. 

NOTE 4 — Items (c), (d) and (e) are of limited 
interest as they can be applied only in special situations 
and used under effective supervision. They all include 
a high degree of isolation from earth. 

NOTE 5 — In this Section, detailed consideration is 
limited to earthed equipotential bonding and automatic 
disconnection of the supply. 

18.2.2 Earthed Equipotential Bonding and Auto- 
matic Disconnection of the Supply — - The two aims of 
this protective measure are to: 

a) ensure that when an earth fault occurs, the 
voltages appearing between exposed con- 
ductive parts and extraneous conduetive 
parts in the location served by the installa- 
tion concerned are minimized; and 

b) ensure rapid disconnection of the circuit in 
which that earth fault occurs. 



In order to meet (a), a zone is created by 
first connecting all extraneous conduetive parts by 
means of equipotential bonding conductors to the 
main earthing terminal or earth electrode(s) of 
the installation. 

The zone is completed by the connection of 
all exposed conduetive parts of the circuits in the 
installation and of current-using equipment fed 
from those circuits to the main earthing terminal 
(or installation earth electrode) using circuit 
protective conductors. 

Whilst such a zone is called an equipotential 
zone, this does not mean that voltages cannot 
exist between conductive parts in that zone when 
an earth fault occurs. The voltages referred to 
earlier {see 18.1) will still exist between the 
exposed conduetive parts of perfectly sound equip- 
ment and between such parts and extraneous 
conductive parts, but the application of bonding 
minimizes these voltages in each case. 

An installation may consist of a number of 
zones; for instance, when an installation supplies 
a number of buildings, equipotential bonding is 
necessary in each building so that each constitutes 
a zone having a reference point to which the 
exposed conductive parts of the circuits and 
current-using equipment in that building are 
connected. 

The second aim of this protective measure is 
met by limiting the upper value of the earth fault 
loop impedance of each circuit to a value deter- 
mined by the type and current rating of the pro- 
tective device concerned such that, on the 
occurrence of an earth fault (assumed to be of 
negligible impedance), disconnection will occur 
before the prospective touch voltage reaches a 
harmful value. 

18.2.3 Extraneous Conductive Parts — The extra- 
neous conduetive parts that are required to be 
bonded to the main earthing terminal of the 
installation (or to the earth electrode of the 
installation) include: 

a) gas pipes; 

b) other service pipes and ducting; 

c) risers and pipes of fire protection equip- 
ment; 

d) exposed metallic parts of the building 
structure; and 

e) lightening conductors {see Section 8). 

NOTE — Connections to pipes, ducting and exposed 
metallic parts of building structure shouldbe considered 
most carefully. In some types of earthing systems, 
especially TN-C orTN-C-S systems effectively connect 
extraneous conducting metalwork to the supply system 
neutral and could cause continuously circulating 
currents and standing voltages that might result in 
electrochemical corrosion or random spark hazards in 
potentially flammable atmospheres. 



36 



18:3043-1987 



18.2.4 Exposed Conductive Parts — Exposed con- 
ductive parts that are required to be connected 
by means of protective conductors to the main 
earthing terminal (or earth electrode) of the 
installation are as follows: 

a) All metalwork associated with wiring sys- 
tem (other than current-carrying parts) 
including cable sheaths and armour, con- 
duit, ducting, trunking, boxes and catenary 
wires; 

b) The exposed metalwork of all Class I fixed 
and portable current-using equipment. 
Even where at the time of the erection of 
the installation this equipment is of Class II 
construction or its equivalent, because 
there is a possibility that in the life of the 
installation the equipment maybe replaced 
by Class I equipment, all fixed wiring 
accessories should incorporate an earthing 
terminal that is connected to the main 
earthing terminal by means of the protec- 
tive conductors of the circuits concerned. 

c) The exposed metalwork of transformers 
used in the installation other than those 
that are an integral part of equipment. 
The secondary windings of transformers 
should also be earthed at one point of the 
winding, unless the transformer is a safbty 
isolating transformer supplying a part of 
the installation where the protective 
measure 'electrical separation' is being 
used). 

Exposed conductive parts that (because of 
their small dimensions or disposition) cannot be 
gripped or contacted by a major surface of the 
human body (that is, a human body surface not 
exceeding 50 mm x 50 mm) need not be earthed 
if the connection of those parts to a protective 
conductor cannot readily be made and reliably 
maintained. Typical examples of such parts are 
screws and nameplate, cable clips and lamp caps. 
Fixing screws for non-metallic accessories need 
not be earthed provided there is no appreciable 
risk of the screws coming into contact with live 
parts. 

Other exposed conductive parts not required 
to be earthed are: 

1) Overhead line insulator brackets and metal 
parts connected to them if such parts are 
not within arm's reach; and 

2) Short lengths of metal conduit or other 
metal enclosures used to give mechanical 
protection to equipment of Class II or 
equivalent cunstruction. 

18. Protection against Excessive Tempera- 
ture Rise and Mechanical Damage 

18.3.1 General — The protective circuit of an 
installation includes the following {see Fig. 20): 

a) Circuit protective conductors; 



b) Equipotential bonding conductors; and 

c) Earthing conductors. 

Under certain circumstances, there may also 
be local equipotential bonding conductors. 

The determination of cross-sectional areas of 
all these conductors is the subject of Section 2 
(also see 18.4) and here consideration is limited 
to the types of conductor that can be used with 
some indication of the precautions that should be 
taken during erection, particularly those concern- 
ed with mechanical and chemical deterioration 
and electro-dynamic effects. 

18.3.2 Earthing conductors — Copper earthing 
conductors, in general, need not be protected 
against corrosion when they are buried in the 
ground if their cross-sectional area is equal to or 
greater than 25 mm^. In case of buried steel con- 
ductors, appropriate corrosion factors based upon 
the summed up corrosion indexes corresponding 
to different parameters connected with the mate- 
rial for grounding, environmental conditions, 
nature of soil, etc (see Section 4) should be 
applied in determining the size of the earthing 
conductor, however, the minimum size should not 
be less than 50 mm". If the earthing conductor is 
of tape or strip, the thickness should be adequate 
to withstand mechanical damage and corrosion. 

It should be remembered that plain uncoated 
copper is positive to plain uncoated buried steel 
and when interconnected by a current carrying 
conductor, these metals will form an electroche- 
mical cell that can cause accelerated corrosion of 
steel. As a rough guide, a dc current of 1 A leav- 
ing a buried steel structure can remove nearly 
9 kg of metal in one year. 

Where such conductors are protected against 
corrossion but are not mechanically protected, the 
minimum cross-sectional area is 16 mm' if the 
conductor is of copper or coated steel (Table 4). 
The determination of the cross-sectional area 
where the earthing conductor is both mechani- 
cally protected and protected against corrosion is 
considered in a later section. 

Aluminium or copper clad aluminium conduc- 
tors should not be used for final underground 
connections to earth electrodes. Where a copper 
conductor is to be joined to aluminium, the 
copper should be tinned, unless an approved con- 
ductor is used. 

The connection of the earthing conductor to 
the earth electrode or other means of earthing 
should be readily accessible and soundly made by 
the use of soldered joints or substantial clamps of 
non-ferrous material. Where the earthing conduc- 
tor is to be connected to the metal armour and 
sheath of a cable, the armour should be bonded 
to the metal sheath and the principal connection 
between the cable and the earthing conductor 



37 



IS : 3043 - 1987 



should be to the metal sheath, and should prefe- 
rably be soldered. However, if a clamp is used for 
this connection the clamp should be so designed 
and installed as to provide reliable connection 
without damage to the cable. 

18.3.3 Circuit Protective Conductors — A circuit 
protective conductor may form part of the same 
cable as the associated live conductors, either as a 
core of that cable or the metallic sheath or arm- 
ouring, or it may be separately run insulated 
conductor, the insulation being at least equivalent 
to that provided for a single core non-sheathed 
cable of appropriate size. A separately run circuit 
protective conductor having a cross-sectional area 
greater than 6 mm^ or of copper strip is not 
required to be insulated. All protective conductors 
should, however, be protected against physical 
damage and other forms of damage, for example, 
welding current stray return paths. Where the 
sheath of a cable incorporating an uninsulated 
protective conductor having a cross-sectional area 
of 6 mm" or less is removed at joints and the 
termination, the conductor should be protected 
by insulating sleeving. 

When the metallic sheath is used every joint in 
that sheath should be so made that its current 
carrying capacity is not less than that of the 
sheath and where non-metallic joint boxes are 
used, means such as a metal strip having a resis- 
tance not greater than that of the corresponding 
length of sheath of the largest cable entering the 
box should be provided to maintain continuity. 

When using the metallic sheath or armour as 
a protective conductor, attention should be paid 
to the ability of cable glands and connections to 
carry prospective earth fault currents. Particular 
care should be taken to avoid problems with non- 
conducting finishes. 

Metallic enclosures for cables, such as conduit, 
ducting and trunking, may be used as circuit pro- 
tective conductors but where flexible or pliable 
conduit is used, separate protective conductors 
should be used to maintain the integrity of the 
earth path. Where conduit is used, a high standard 
of workmanship in installation is essential. Joints 
should be so made that their current carrying 
capacity is not less than the conduit itself Slack- 
ness in joints can result in deterioration in and 
even complete loss of continuity. Plain slip or pin- 
grip sockets are considered insufficient to ensure 
satisfactory electrical continuity of joints. In the 
case of unscrewed conduit, the use of lug-grip fitt- 
ing is recommended, but for outdoor installations 
and where otherwise subjected to atmosphere 
corrosion, screwed conduit should always be used, 
suitably protected against corrosion. In screwed 
conduit installations, the liberal use of locknuts is 
recommended. Joints in all conduit systems 
should be painted overall after assembly. 

These precautions should be adequate, but 
periodical tests should be made to verify that 
electrical continuity is satisfactorily maintained. 



18.3.4 Local Equipotentlal Bonding (18.0, 10) — 
The equipotential zone partially created by the 
bonding of extraneous conductive parts to the 
main earthing terminal depends for its efficacy on 
metal-to-metal contact of negligible impedance. 
Within a particular part of the zone where extra- 
neous conductive parts are simultaneously accessi- 
ble with either other extraneous conductive parts 
or exposed conductive parts or both, tests may 
show that it is necessary to carry out local equipo- 
tential bonding between the parts concerned in 
order to obtain satisfactory low impedance. 

18.3.5 Etectrolylic Corrosion — Under damp con- 
ditions, electrolytic corrosion is liable to occur at 
contacts between dissimilar metals. Copper and 
alloys having a high copper content are particu- 
larly liable to cause corrosion under these condi- 
tions when in contact with aluminium based 
alloys. 

When disimilar metals form part of an electri- 
-al circuit, the joints should be clean and 
assembled free of moisture, and then immediately 
sealed with a suitable medium against the ingress 
of moisture. 

Where damp conditions prevail, the fittings, 
fixing screws and saddles used to secure alumi- 
nium based alloy conductors, should be made of 
aluminium alloy or suitably protected steel (zone 
coated) and all the points of contact between 
them painted. 

Particular attention should be paid to pipe- 
work because of the risk of replacement of part of 
the pipe system by non-metallic pipes or joints. 
Metalwork that may require bonding includes 
exposed metal pipes, sinks taps, tanks, radiators, 
and where practicable and accessible, structural 
components. 

18.4 Cross-Sectional Areas of the Conductors 
of an Installation Protective Circuit — The 

cross-sectional areas of the conductors of the pro- 
tective circuit are influenced by the limitation, 
placed on earth loop impedances to ensure discon- 
nection of the circuit in which and earth fault 
occurs in the prescribed time, that is, instantane- 
ous disconnection for higher control potential and 
disconnection with time lag for lower voltages. 

Where a protective device concerned is a fuse, 
miniature circuit breaker or other types of series 
over-current device, those disconnecting times 
imply that the earth fault loop impedances should 
be such that the earth fault current is considera- 
bly greater than the rated current of the device 
(or of the same order as occurring under short- 
circuit conditions) Residual Current Devices 
(RCDs) shall be provided to disconnect the cir- 
cuit within the same time in case of impedance or 
arcing fault conditions. The device setting should 
be interlinked with earth fault loop impedance, 
safe contact potential and permissible time for 
disconnection. 



38 



IS : 3043 - 1987 



All the constituent conductors of the protective 
circuit should ' therefore be of adequate cross- 
sectional area to ensure that the temperatures 
attained by the conductors do not exceed their 
prescribed limiting values. 

18,5 Consumers' Earth Connections (see 6.1.1) 
— The method of connection of the main 
earthing terminal of an installation to earth 
depends on the type of system of which that 
installation is part. The different systems are 
described in Fig. 2 to 8. 

When the source of energy is privately owned, 
there should be no metallic connection with the 
general public supply unless there has been con- 
sultation with the electricity authority concerned. 

It should be emphsized that an installation 
together with its source of energy may not consist 
entirely of one particular type of system. In such 
cases, each part of that installation may be 
required to be treated separately without detri- 
ment to other parts of the same installation. By 
and large, the types of system encountered fall in 
one or other categories shown in Fig. 2 to 8. 

19. SELECTION OF DEVICES FOR 

AUTOMATIC DISCONNECTION OF 
SUPPLY 

19.1 General — In general, every circuit is pro- 
vided with a means of overcurrent protection. If 
the earth fault loop impedance is low enough to 
cause these devices to operate within the specified 
times (that is, sufficient current can flow to earth 
under fault conditions), such devices may be 
relied upon to give the requisite automatic discon- 
nection of supply. If the earth fault loop impe- 
dance does not permit the overcurrent protective 
deviecs to give automatic disconnection of the 
supply under earth fault conditions, the first option 
is to reduce that impedance. It may be permi- 
ssible for this to be, achieved by the use of protec- 
tive multiple earthing or by additional earth 
electrodes. There are practical limitations to both 
approaches. 

In case of impedance/arcing faults, series pro- 
tective devices may be ineffective to clear the 
faults. An alternate approach is to be adopted for 
the complete safety of the operating personnel 
and equipment from the hazards that may result 
from earth faults. This is to use residual current 
devices with appropriate settings to clear the faults 
within the permissible time, based on the probable 
contact potential. This method is equally applica- 
ble where earth loop impedances cannot be 
improved. 

In TT systems, there is an additional option 
of the use of fault voltage operated protective 
devices. Whilst these devices will always give pro- 
tection against shock risk, provided they are 
correctly installed, the presence of parallel earths 
from the bonding will reduce the effectiveness of 
the fire risk protection they oft^r. These are. 



therefore, more suited for isolated installations 
that do not have interconnections to other insta- 
llations. It should also be remembered that every 
socket outlet circuit that do not have earthing 
facility in a household or similar installation 
should be protected by a residual current device 
having a rated residual operating current not 
exceeding 30 mA. 

On all other systems where equipment is 
supplied by means of a socket outlet not having 
earthing facility or by means of a flexible cable or 
cord used outside the protective zone created by 
the main equipotential bonding of the installation 
such equipment should be protected by a residual 
current operated device having an operating cur- 
rent of 30 mA or less. 

19.2 Use of Overcurrent Protective Devices 
for Earth Fault Protection — Where over- 
current protective devices are used to give auto- 
matic disconnection of supply in case of earth fiiult 
in order to give shock risk protection, the basic 
requirement is that any voltage occurring between 
simultaneously accessible conductive parts during 
a fault should be of such magnitude and duration 
as not to cause danger. The duration will depend 
on the characteristic of the overcurrent device and 
the earth fault current which, in turn, depends on 
the total earth fault loop impedance. The magni- 
tude will depend on the impedance of that part of 
the earth fault loop path that lies between the 
simultaneously accessible parts. 

The basic requirement can be met if: 

a) a contact potential of 65 volts is within the 
tolerable limits of human body for 
10 seconds. Hence protective relay or 
device characteristic should be such that 
this 65 volts contact potential should be 
eliminated within 10 seconds and higher 
voltages with shorter times. 

b) a voltage of 250 volts can be withstood by 
a human body for about 100 milli seconds, 
which requires instantaneous disconnection 
of such faults, giving rise to potential rise 
of 250 volts or more above the ground 
potential. 

The maximum earth fault loop impedance 
corresponding to specific ratings of fuse or minia^ 
ture circuit breaker that will meet the criteria can 
be calculated on the basis of a nominal voltage to 
earth (Uo) and the time current characteristics 
of the device assuming worst case conditions that 
is, the slowest operating time accepted by the 
relevant standards. Thus, if these values are not 
exceeded, compliance with this code covering 
automatic disconnection in case of an earth fault 
is assured. 

Where it is required to know the maximum 
earth fault loop impedance acceptable in a circuit 
feeding, a fixed appliance or set of appliances and 
protected by an over current device, the minimum 



39 



IS : 3043 - 1987 



current that may be necessary to ensure operation 
of the overcurrent device within the permissible 
time of 10 seconds for a contact potential of 
65 volts is found from the characteristic curve of 
the device concerned. Application of the Ohm's 
Law then enables the corresponding earth fault 
loop impedance to be calculated as provided in 
the formulae in 18.0.3 to 18.0.6. 

For circuits supplying socket outlets, the 
corresponding earth fault loop impedance can be 
found by a similar calculation for earthed equip- 
ment. When equipment are not earthed and con- 
nected to socket outlets without earthing facility, 
disconnection should be ensured for 30 niA with- 
in 10 seconds and with appropriate decrements In 
time for higher currents. 

This method requires a knowledge of the total 
earth loop impedance alone (rather than indivi- 
dual components) and is, therefore, quick and 
direct in application. Its simplicity does exclude 
some circuit arrangements that could give the 
required protection. 

While calculations give the maximum earth 
fault loop or protective conductor impedance to 
ensure shock risk protection under fault conditions 
it is also necessary to ensure that the circuit pro- 
tective earth conductor is protected against the 
thermal effects of the fault current. The earth 
fault loop impedance should, therefore, be low 
enough to cause the protective device to operate 
quickly enough to give that protection as well. 
This consideration places a second limit on the 
maximum earth loop impedance permissible and 
can be checked by superimposing on the time 
current characteristic of the overload device, the 
'adiabatic' line having the equation: 



tmm 



or A 



/i -. .- - ^ 

NOTE — Values of k for typical protective conduc- 
tor conditions are given in 12.2.2.1 and Tables 6A to 
6D. 

Details of the maximum permissible earth loop 
impedance for the thermal protection of cables by 
fuses can also be computed. However, the time 
current characteristics of a miniature circuit 
breaker are such that if the loop impedance is low 
enough to give automatic disconnection within 
safe disconnecting time so providing shock risk 
protection, it will also give the necessary thermal 
protection to the earth conductor likely to be used 
with a breaker of that specific rating. Figure 21 
shows the relationship between the adiabatic line 
and the characteristic of fuses and miniature 
circuit breaker. 

In order that the devices will give thermal 
protection to the protective conductor, operation 
has to be restricted to the area to the right of 
point A where these curves cross. Thus, the maxi- 
mum earth fault loop impedance for thermal 
protection of the cable is that corresponding to 
the minimum earth fault current for which the 
device gives protection. The value of this current 



can be read from the curve and the corresponding 
loop impedance can be calculated from: 



where 

^s = earth fault loop impedance, 

IJ^^ = nominal voltage to earth, and 

/r = earth fault current. 

For a given application, the maximum permit- 
ted earth fault loop impedance would be the lower 
of the two values calculated for shock risk protec- 
tion or thermal restraint respectively. 

It will be noted that the adiabatic line crosses 
the characteristic curve for a miniature circuit 
breaker at a second point B. This denotes the 
maximum fault current for which a breaker will 
give thermal protection but it will generally be 
found in practice that this value is higher than the 
prospective short circuit current that occurs in the 
circuit involved and cannot, therefore, be 
realized. 



FUSE CHARACTERISTiCS 




AOiAiAKC UHE 



I 

i 



CUMENf J &^ A 

21 A Fuses 



/CH^RACIERlSTfCS 




CURRENT I 9^ 

21 B Miniature Current Breaker 
FIG. 21 RELATIONSHIP BETWEEN ADIABATIC 
LINES AND CHARACTERISTICS 



40 



IS : 3043 - 1987 



19.3 Earth Fault Protective Devices — There 
are two basic forms of such devices that can be 
used for individual non-earthed/carthed (with 
limited application) equipment as follows; 

a) Residual Current Operated Devices (RCD) — An 
RCD incorporates two component items. A 
core balance transformer assembly with a 
winding for each recognizing the out of 
balance current that the fault produces in 
the main conductors. This induces a current 
that is used to operate the tripping mecha- 
nism of a contact system. For operating 
currents of 0-5 A or more, the output from 
such a transformer assembly can operate a 
conventional trip coil directly. For lower 
values of operating current, it is necessary 
to interpose a leay device, either magnetic 
or solid state. 

Devices for load currents greater than 
100 A usually comprise a separate transfor- 
mer assembly with a circuit breaker or con- 
tact relay, mounted together within a 
common enclosure. Devices for load 
currents below 100 A usually include the 
transformer and contact system within the 
same single unit, which is then described as 
a residual current operated circuit breaker 
(RCB). Such an RCB should be considered 
a particular type of RCB although it is the 
most usual form. 

A wide choice of operating currents is 
avilable (typical values are between 10 mA 
and 20 A) RGB's are normally non- 
adjustable whilst RCD's are often manufac- 
tured so that one of several operating 
currents may be chosen. Single phase and 
multiphase devices with or without integral 
overcurrent facilities are available. 

Where residual current breakers of 30 mA 
operating current or less are being used, 
there is a choice between devices that are 
entirely electromechanical in operation and 
those that employ a solid state detector. 
The electromechanical types are generally 
small and compact and will operate on the 
power being fed to the fault alone whereas 
the solid state type which tend to be bulkier 
to require a power supply to ensure opera- 
tion. Where this power supply is derived 
from the mains, it may be necessary to take 
added precaution against failures of part of 
that mains supply. Devices suitable for time 
grading are more likely to be of the solid 
state form as are those having higher 
through fault capacity. 

A test device is incorporated to allow the 
operation of the RCD to be checked. 
Operation of this device creates an out of 
balance condition within the device. Tripp- 



ing of the RCD by means of the test device 
establishes the following; 

1) the integrity of the electrical and 
mechanical elements of the tripping 
device; and 

2) that the device is operating at appro- 
ximately the correct order of operat- 
ing current. 

It should be noted that the test device does 
not provide a means of checking the conti- 
nuity of the earthing lead or the earth 
continuity conductor, nor does it impose 
any test on the earth electrode or any other 
part of the earthing circuit. 

Although an RCD will operate on currents 
equal to or exceeding its operating current, 
it should be noted that it will only restrict 
the time for which a fault current flows. It 
can not restrict the magnitude of the fault 
current which depends solely on the circuit 
conditions. 

b) Fault Voltage Operated Earth Leakage Circuit 
Breakers (ELCB) — A voltage operated 
earth leakage circuit breaker comprises a 
contact switching system together with a 
voltage sensitive trip coil. On installations, 
this coil is connected between the metal- 
work to be protected and as good a con- 
nection with earth as can be obtained. Any 
voltage rise above earth on that metalwork 
exceeding the setting of the coil will cause 
the breaker to trip so giving indirect shock 
risk protection. 

Tripping coils are designed so that a fault 
voltage operated device will operate on a 
40 V rise when the earth electrode resis- 
tance is 500n or 24 V on a 200n electrode. 
Single and multiphase units, with or with- 
out overcurrent ifacilities, are available for 
load currents up to 100 A. 

A test device is provided on a voltage 
operated unit to enable the operation of 
the circuit breaker to be checked, operation 
of the device applies a voltage to the trip 
coil so simulating a fault. 

Tripping of the circuit breaker by means 
of the test device shows the integrity of the 
electrical mechanical elements that the unit 
is operating with the correct order of ope- 
rating voltage and, in addition, proves the 
conductor from the circuit breaker to the 
earth electrode. It can not prove other 
features of the installation. 

Whilst the voltage operated (ELCB) will 
operate when subjected to a fault voltage 
of 20 V or more, it should be noted that it 
cannot restrict the voltage in magnitude 
only in duration. 



41 



IS : 3043 - 1987 



c) Current Operated Earth Leakage Circuit Breakers — 
For industrial applications, earth leakage 
circuit breakers operating on inilliampere 
residual currents or working on fault volt- 
age principle are of little use, since milli- 
amperes of earth leakage current for an 
extensive industrial system is a normal 
operating situation. Tripping based on 
these currents will result in nuisance for the 
normal operation. Milliamperes of current 
in a system, where exposed conductive 
parts of equipments are effectively earthed 
and fault loop impedance is within reason- 
able values, will give rise only to a ground 
potential/contact potential rise of a few^ 
millivolts. This will in no way contribute to 
shock or fire hazard. Here objectionable 
fault currents will be a few or a few tenths 
of amperes. In such cases, residual current 
operated devices sensitive to these currents 
must be made use of for earth fault current 
and stable operation of the plant without 
nuisance tripping. This is achieved either 
by separate relays or in-built releases initi- 
ating trip signals to the circuit-breakers 
(For details, refer to Section 5). 

19.4 Selection of Earth Fault Protective 
Devices — In general, residual current operated 
devices are preferred and may be divided into two 
groups according to their fmal current operating 
characteristics. 

a) RCD's Having Minimum Operating Currents 

Greater Than 30 mA — These devices are 
intended to give indirect shock risk protec- 
tion to persons in contact with earthed 
metal. 

b) RCD's Having Minimum Operating Current of 

30 mA and Below — These devices are gene- 
rally referred to as having 'high sensitivity' 
and can give direct shock risk protection to 
persons who may come in contact with live 
conductors and earth provided that the 
RCD operating times are better than those 
given in IS : 8437-1977*. It should be 
noted that such RCD's can only be used to 
supplement an earth conductor and not 
replace one. 

In addition to giving protection against 
indirect contact or direct contact RCD's may also 
give fire risk protection, the degree of protection 
being related to the sensitivity of the device. 

An RCD should be chosen having the lowest 
suitable operating current The lower the operat- 
ing current the greater the degree of protection 
given, it can also introduce possibilities of nuisance 
tripping and may become unnecessarily expensive. 



*Guide on eifeds of current passing through the human 
body. 



The minimum operating current will be above 
any standing leakage that may be unavoidable on 
the system. A further consideration arises if it is 
intended to have several devices in series. It is not 
always possible to introduce time grading to give 
discrimination whereas a limited amount of current 
discrimination can be obtained by grading the 
sensitivities along the distribution chain. 

The maximum permitted operating current 
depends on the earth fault loop impedance. The 
product of the net residual operating current loop 
impedance should not exceed 65 volts. 

It is often acceptable on commercial grounds 
to have several final circuits protected by the same 
residual current devices. This, however, does 
result in several circuits being affected if a fault 
occurs on one of the circuits so protected and the 
financial advantages have to be weighed against 
the effects of loosing more than one circuit. 

It should also be noted that different types of 
RCD in different circuits may react differently to 
the presence of a neutral to earth fauh on the load 
side. Such an earth connection together with the 
earthing of the supply at the neutral point will 
constitute a shunt across the neutral winding on 
the RCD transformer. Consequently, a portion of 
the neutral load current will be shunted away 
from the transformer and it may result in the 
device tripping. On the other hand, such a shunt 
may reduce the sensitivity of the device and pre- 
vent its tripping even under line to earth fault 
conditions. In general, therefore, care should be 
taken to avoid a neutral to earth fault where 
RCD's are in use, ahhough there are some designs 
being developed that will detect and operate 
under such conditions. On installations with seve- 
ral RCD's, care should be taken to ensure that 
neutral currents are returned via the same device 
that carries the corresponding phase current and 
no other. Failure to observe this point could resuh 
in devices tripping even in the absence of a fault 
on the circuit they are protecting. 

When using fault voltage operated ELCB's, 
the metalwork to be protected should be isolated 
from earth so that any fault current passes through 
the tripping coil gives both shock and fire risk 
protection. However, this isolation is not always 
practicable and the presence of a second parallel 
path to earth will reduce the amount of fire risk 
protection offered. Because the coil is voltage 
sensitive, the presence of such a parallel path will 
not reduce the shock risk protection offered provi- 
ded that this second path goes to earth well clear 
of the point at which the earth leakage circuit 
breaker trip coil is earthed. It is required that the 
earthing conductor is insulated to avoid contact 
with other protective conductors or any exposed 
conductive parts or extraneous conductive parts so 



42 



IS : 3043 - 1987 



as to prevent the voltage sensitivity element from 
being shunted, also the metalwork being protected 
should be isolated from that associated with other 
circuits in order to prevent imported faults. 

Voltage operated ELCB's are suitable for pro- 
tection of isolated installations on a T T system 
such as occur in rural areas. Table 9 shows the 
maximum earth electrode impedance with switch 
different types of breaker may be used. 



TABLE 9 MAXIMUM EARTH ELECTRODE 

RESISTANCE FOR DIFFERENT TYPES OF 

CIRCUIT BREAKER 



TYPE OF BREAKER 



RCD 

Voltage Operated 
ELCB 



OPERATING 
CURRENT 



300 mA 
r 30niA 



MAXIMUM EARTH 
ELECTRODE 

RESISTANCE (Q) 

166 

I 666 

500 



SECTION 4 POWER STATIONS, SUBSTATIONS AND OVERHEAD LINES 



20. EARTHING IN POWER STATIONS AND 
SUBSTATIONS 

20.1 General — In general, earthing installations 
will be required at power stations and substations 
for: 

a) The neutral points of each separate electri- 
city system which has to be earthed at the 
power station or substation; 

b) Apparatus fremework or cladding or other 
non-current carrying metalwork associated 
with each system, for example, translbrmer 
tanks, power cable sheaths; 

c) Extraneous metalwork not associated with 
the powersystems, for example boundary 
fences, sheaths of control or communication 
cables. 

For safety, the objective of earth bonding is to 
ensure that, in normal or abnormal conditions, 
any voltage appearing on equipment to which 
there is access should be below a dangerous level. 
It is not practicable to ensure that metal parts are 
earthed and remain near true earth potential 
during the passage of earth feult currents, particu- 
larly on high voltage systems with directly earthed 
neutrals. The objective should, therefore, be to 
provide effective bonding of low impedance and 
adequate current-carrying capacity between parts 
with which anyone may be in simultaneous con- 
tact, and to arrange, as far as possible, that large 
fault currents do not How between such points. 

To minimize risk of damage to certain auxili- 
ary plant, the rise of potential of a station earthing 
installation above the potential of true or remote 
earth should be as low as practicable, since this 
potential will be applied across protective insulation 
of any plant with connections to earth external to 
the substation, for example, plant with connections 
to pilot or telephone cables or cable sheaths. For 
similar reasons, the potential difference between 
earthed points in the station should also be kept 
to a minimum. Where surge protection is provided, 
the connection of the protective devices to earth 
should be as direct as possible. The discharge of 
high currents with high-frequency components 
requires earth connections of low resistance and 
reactance, that is, short connections with as few 
changes of direction as possible. 



Where the neutral points of two electrically 
separate electricity systems are connected to a 
common earth electrode system at a site, there is 
a couphng of the systems in the event of an earth 
fault occurring on either system by virtue of the 
rise of earth potential due to the passage of the 
fault current through the earth electrode system. 
Similarly, if non-current carrying metalwork is 
bonded to the same earth electrode as the neutral 
point of the supply the metalwork will experience 
the same rise of earth potential. If complete sepa- 
ration of electrical systems were required, it would 
be essential that the neutral points of each system 
and its associated metalwork be separately earthed. 
If such a method were adopted, each earthing 
system would require insulation from other ear- 
thing systems to withstand the maximum rise of 
earth potential occurring in any system by virtue 
of lightning currents or power system fault cur- 
rents. Insulation to this level is rarely practicable. 

The choice of using a common earth or sepa- 
rate earths for the system of different voltages at a 
transforming point affect: 

a) the probability of breakdown occurring in a 
transformer between the higher and lower 
voltage sides due to lighting or other surges; 
and 

b) the safety of consumers or their property 
supplied by any low voltage system distri- 
buted from the station against arise of 
potential of the earthed neutral by a high 
voltage system earth fault at the station. 

The former risk is reduced by use of a common 
earth system, and the latter danger only arises if 
the resistance of the earth electrode system is not 
sufficiently low to limit the rise of earth potential 
to a safe value. There is advantage in using a 
common earth where the earth electrode resistance, 
including the parallel resistance of any bonded 
metalwork, etc, to earth is 1 Q. or less, as is usual 
at power stations, large outdoor substations or sub- 
stations supplying a network of cables whose 
sheaths have a low impedance to earth. 

The substation earth system rise of potential 
will not be excessive if the resistance of the earth 
electrode system is small compared to the total 
earth fault circuit impedance. Systems of higher 



43 



IS : 3043 - 1987 



yoltage (66 kV and abcve) generally have the 
neutral directly earthed, since the increase in costs 
of insulation that would be required for the trans- 
fonner winding would be considerable. 

In rural situations, where overhead hues are 
used, it may, in certain circumstances, be inad- 
visable to use a common earth (see 20.2). 

The requirements are, therefore, best consi- 
dered separately for substations: 

a) where low voltage is confined to auxiliary 
supplies within the substation; 

b) substations that provide an external low 
voltage supply; and 



c) power stations. 

The use of neutral earthing switchgear in public 
supply systems is avoided, where possible, since a 
direct earth is simple, reliable ancl cheaper than a 
switched earth. Tlie circumstances in which neu- 
tral earthing switchgear may be necessary are so 
broad that it is not practicable to fomi general 
rules on type and application. 

20.2 General Earthing Arrangement — A 

typical earthing arrangement for an outdoor 
switchyard is shown in Fig. 22. A typical earthing 
arrangement for connecting the reinforcement of 
foundations of substation building and switchyard 
RCC masts is shown in Fig. 23. 



ipkBmif'iks'^ftS CAtcutirtfo»r^ 



-«»—«- 



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5- 



ly 



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r^ 



i-<^ 



'O 




O 



i> 



<> 






T 

o 



LIdHTNSNO 
ARRESTER 



iC^II^^ 



T — I n 

5 Oi 



®=i 



ISOLATOR 



^^ 



im 



tooc 



nnn 



POTENTIAL 
TAANSrORMER 



nnn 



o 



nnn ^^^*^^^^^'*^^^ nnn 



■c 



X 



1 



CIRCUIT 
8REAKER 



o 



> 



^fr3jng^ 



ISOLATOR 



4=4=^ 



-^^^ 



ISOLATOR 



ifr^frl^ fl ^ - 



nnn twaksformer nnn o 



CIRCUIT 
BREAKER 



Q €) 



liohtning 
arrester 



^« 



o o o 
99% 



I CrvncAL E 






POWER 
^ TRANSFORMER 



"i5'"o"tt"" 



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1 LXi La 



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0;t 



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TYP8CAL iARTH ft RIP 



-7, 



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EARTH *• 

ELECTROOi 

ilYPI 



NOTE — The number of electrodes and the size of the grid conductor is to be worked out as per 22.6.2. 
FIG. 22 ATYPICAL EARTHING GRID FOR AN OUTDOOR SUBSTATION (66 kV AND ABOVE) 



44 



IS : 3043 - 1987 




SECTION AA 



mui 




.ONLY TOP RING TO B£ 
WELOtO TO MAIN REiN- 
rORCEMENT POOS 



EARTHING PAD TOO kIQO 
^(TO 0E CONNECTED TO 
MAIN EARTH GRID) 



SUBSTATION BUILDING 
COLUMN 



NOTE 1 — Top ring should be half the size of main vertical reinforcement rod. 
NOTE 2 - Two extreme columns should be earthed like this in each substation. 
NOTE 3 - This is applicable to RCC masts and equipment supports in CD switchyard. 
NOTE 4 - Inserts other than earthing pads may or may not be welded to reinforcement. 

FIG. 23 EARTHING OF FOUNDATION REINFORCEMENT (CONCRETE ENCASED 
EARTHING ELECTRODE) 



The perimeter fence may need to be earthed 
separately from the main station earth electrode 
system (see 20,6.1). 

The tertiary winding of a power transformer 
should be connected to the transformer tank by a 
connection of sufficient cross-sectional area to carry 
the primary short-circuit current. 

In the case of pole mounted transformers on 
overhead line systems, difficulties may arise in 
areas of high soil resistivity. Here, if the pole car- 
ries also isolating switchgear with low level operat- 
ing handle, up to three separately earthed elec- 
trode systems may be required. That for the neutral 
of the low voltage system is usually provided not 
nearer than one pole span away on the low voltage 
line. That for the high voltage metal work (trans- 
former tank, switch framework, support metal 
work), consists of one earth electrode at or near 
the pole. Resistances of 5 to 50 Q are sometimes 



the minimum economically possible. In audition, 
an earth mat should be provided, near the ground 
surface, in the position taken up by a person opera- 
ting the switch handle; this mat should be con- 
nected to the switch handle. The mat should be 
electrically separated from the main electrode; this 
is considered to be achieved by spacing the J 
nearest element of that electrode at least 1 m from 
the periphery of the mat and by placing the two 
earthing-wires on opposite sides of the pole. The 
tops of the main electrodes should be at least 225 
mm and preferably 750 mm below the ground, 
and the earthing wire to the main electrode of 
outdoor type rubber or plastics-insulated cable up 
to a point 2 m above ground level. This cable, 
between the bottom of the pole and the electrode 
should be laid in a 50-mm diameter earthenware 
duct filled solid with bitumen. 
20.3 General Earthing Arrangements at 
Power Stations of Public Electricity Sup- 
plies 



45 



IS : 3043 - 1987 



20.3.1 Neutral Earthing of Generator Circuits — At 
modern large power stations for public electricity 
supply the generation circuits generally comprise 
a star-connected stator circuit with an operating 
voltage up to about 26 kV, directly connected 
to a step-up delta/star transformer, the higher 
voltage winding generally operating at 132 Vk, 
275 kV or 400 kV, with the transmission system 
neutral point directly earthed. 

The following three methods have been used 
for earthing the neutral of the generator windings: 

a) Earthing through the primary winding of a 
matching transformer, with resistor connec- 
ted across the secondary winding; 

b) earthing through a resistor; and 

c) earthing through the primary winding of 
a voltage transformer. 

Method {a) — is current practice, the design be- 
ing such that the maximum substained earth fault 
current in the generator circuit is restricted to 10 
to 15 A, thus limiting the damage at the point of 
fault. The neutral and earthing connections, how- 
ever, are of adequate capacity to withstand for 
3 s the earth fault current that would flow in the 
event of the matching transformer terminals flash- 
ing over during an earth fault. The resistor used 
tor the arrangement is of the metallic grid non- 
inductive type. 

Method (b) — can be used to achieve the same 
degree of fault-current limitation, by design of a 
suitable high-current resistor, but is not preferred 
on the grounds of cost and its less robust construc- 
tion than that of the equipment used in method 
(a). It was earlier practice, however, to individu- 
ally earth each generator at power stations by 
liquid earthing resistors designed to limit the earth- 
fault current to about 300 A. 

Method (c) — is now historic, but had the advan- 
tage that minimal damage resulted at an earth 
fault. If desired, the generator could remain in 
circuit while operational arrangements were made 
to permit its withdrawal. However, this imposed a 
higher voltage stress on the stator windings and 
plant on the unfaulted phases, and the machine 
design usually imposed limitations on this. The 
output from the secondary winding of the voltage 
transformer could be arranged to activate an 
alarm or trip the generator circuit as desired. In 
designing the neutral and earthing connections to 
the voltage transformer, the earth-fault current 
used was that resulting by tlashover of the voltage 
transformer during an earth fault. 

Some old power stations have generators con- 
nected directly to distribution system busbars; in 
general, the neutral terminals of such generators 
have been earthed via liquid neutral earthing resis- 
tors of such a value that the maximum sustained 
earth fault current is of the order of full load cur- 
rent of the generator. Installations of neutral point 
switchboards with switching of neutral points and 



earthing resistors have been abandoned in favour 
of individual unswitched earthing resistors. 

20.3.2 Earthing of Power Station A iixiliary Systems — 
There are, in common use, three methods of 
earthing the neutral point in power station auxili- 
ary systems: 

a) Solid earthing; 

b) earthing through a voltage transformer (or 
voltage relay) with a surge diverter (but 
not a fuse) shunting the primary winding 
(or the relay); 

c) Ilesistance earthing. 

Methods (a) and (c) involve the automatic dis- 
connection of the individual fault circuit. 

With method (b), an alarm can be arranged 
to be operated from the secondary of the voltage 
transformer and the scheme enables all auxiliaries 
to be kept in service until it is convenient to make 
the auxiliary switchboard dead. 

Method (a) is normally used in power stations 
with smaller generating sets and method (c) used 
in the larger power stations. Method (b) has cer- 
tain disadvantages, such as the complication in 
arranging for speedy identification of the indivi- 
dual faulty circuit and the possible difficulties 
arising from functioning of the surge diverter. 

20.4 Equipment Earthing at Power Stations — 

Practice in equipment earthing at power stations 
is identical to that for large substations not giving 
external low voltage supplies (see 20.2). A 
common earth is used for the neutral earthing of 
generators and power station auxiliaries, and for 
all equipment framework, cladding, power cables 
sheaths and extraneous metalwork not associated 
with the power systems, other than the perimeter 
fence (see 20.6.1.). 

20.5 Power Station and Substation Eartb 
Electrodes 

20.5.1 General — The required characteristics 
of earth electrode system are: 

a) a suitably low resistance, under all variations 
due to climatic conditions, for the fault 
currents envisaged; 

b) current carrying capability for all currents 
and durations that may arise in normal 
operating conditions or during fault or surge 
discharge conditions, without undue in- 
crease in resistance; 

c) suitable location in the vicinity of any light- 

ing discharge devices such that earth con- 
nection conductors from such devices are as 
short and straight as possible to minimize 
surge impedance; and 

d) earth electrode installations should be dura- 
ble and of such material and design to 
avoid corrosions. 



46 



IS : 3043 - 1987 



For high voltage system earthing, the value of 
the resistance of the earth electrode system, with 
any adventitious earths due to the bonding of 
metalwork, etc, in contact with earth, should be 
such that the rise in potential of the electrode 
system above the potential of remote earth is as 
low as economically possible. In the absence of 
any specific restriction, attempt should be made 
to restrict the rise of potential within safe value. 
At some sites, the rise in earth potential will in- 
evitably exceed these values, and special pre- 
cautions are necessary. 

Where the soil of a site is hostile by virtue of 
alkalinity or acidity it may be necessary to embed 
earth electrodes in rammed neutral soil to avoid 
corrosion. 

Earth electrode systems can also represent 
some hazard to adjacent underground services or 
structural steelwork through electrolytic action 
between dissimilar metals (see 23). Where this 
danger cannot be avoided by selection of compati- 
ble metals, the adoption of cathodic protection or 
other remedical action may be necessary. 

At power stations and substations the steel 
inforcement in foundations and piles can be used 
to provide an effective electrode system, without 
necessity to provide further buried electrodes. 
Where piles are used they should be bonded by 
welding and connected to earth bonding bars at 
least four points. 

Where no substantial adventitious earths exist 
or where they are in adequate, it is necessary 
to install electrodes (see 9.1, 9.2 and 12.1.1). 

All cladding or steel work at a station should 
be bonded to the earthing system as should all 
structural steel work, but attention is drawn to 
precautions against undue reliance on the latter as 
an electrode. 

20.5,2 Choice and Design — Where electrodes 
of large surface area are necessary to provide the 
requisite current carrying capacity, earth plates are 
recommended. These are generally of cast-iron, 
not less than 12'5 mm thick, and are usually 1-22 m 
by 1-22 m. As an alternative to plates, cast iron 
pipes may be installed. These are, for example, 
about 100 mm in diameter and 3 m long, but are 
not generally as cost-effective as plates for equi- 
valent surface area. 

For lower current rating requirements, driven 
rods are preferred, usually, of the copper-clad steel 
type. They are generally driven in groups, prefer- 
ably with a spacing of not less than their length, 
although this is not always achievable. Closer 
spacing reduces their effectiveness. The use of dri- 
ven rods is advantageous where the deeper stratas 
of a site have a lower resistivity than the upper 
stratas but they may not be suitable if the site is 
stony or has a rock sub-strata. 



At large substation compounds, it is usual to lay 
a mesh of underground earth strips to which system 
neutral terminals and the earth bonding conduc- 
tors from above-ground structures are connected. 
In addition to providing an approximately equi- 
potential surface over the substation, the earth 
strip mesh frequently suffices to provide an elec- 
trode of suitable resistance and current carrying 
capacity without augmentation. 

20.6 Earthing Conductors for Power Stations 
and Substations 

20.6.1 Disposition — It is necessary to provide 
permanent and substantial connections between all 
equipment and the earth electrodes so as to afford 
a low resistance path for fault currents both to 
earth and between items of equipment. In addi- 
tion, all other metal plant in or about the station 
should be connected to the main station earthing 
system. The most efficient disposition of earthing 
conductors required will depend on the layout of 
equipment and the following may be taken as a 
guide; 

a) Indoor Equipment — A main earth bar should 
be provided and connected to the frame- 
work of each item and to the earth-elec- 
trodes. Except for the smallest installations, 
there should be a connection to the earth 
electrodes at each end of the earth bar or, 
if this is in the form of a ring, at several 
points on the ring. These Connections may, 
depending on the layout be buried cables 
of a size adequate for the short-circuit 
current. Where the structure of a switch- 
board is extensive or occupies more than 
one floor, a further parallel main earth bar 
may be required which should be cross- 
connected to its companion bar at one point 
at least in each section of the switchboard. 

The main earthbar should be so placed 
that cable sheaths can be readily connected 
to it. When cables are so connected, the 
bonds should be made to the cable gland 
on which the lead sheath should be plumb- 
ed and the armouring clamped. The main 
earth bar should be accessible for the 
connection of any detachable earthing 
devices provided with the switchgear. 

Branch connections from the main earth 
bar should be provided to all accessory 
equipment, such as control and relay 
panels, constructional steelwork and fire- 
extinguishing equipment. 

Where busbar protection is effected at 
switchboards by frame leakage, two main 
earth bars are required. The frame bar 
interconnecting the framework of the switch 
units will be connected to the true earth 
bar through a current transformer and 
bolted hnks for test purposes. The true earth 
bar should be run separately from the frame 
earth bar in convenient position for the 



47 



IS : 3043 - 1987 



connection of cable sheaths and earthing 
devices. Where it is mounted on the switch 
units, it should be insulated therefrom by 
insulation capable of withstanding a test 
voltage of 4 kV rms alternating current 
for 1 minutes. 

Where insulated cable glands are used, 
it is recommended that 'island' insulation 
should be provided to facilitate testing. 

b) Outdoor Equipment (Excluding Pole Mounted 
Transformers) — A main earth bar should 
be provided, so disposed as to allow of the 
shortest subsidiary connections to all major 
equipment, such as transformers or circuit 
breakers. Wherever possible, this should be 
arranged to form a ring round the station. 
The main earth bar (or ring) should be 
connected where required to earth elec- 
trodes. For larger stations, the ring should 
be reinforced by one or more cross-connec- 
tions. 

From the main earth bar, branch con- 
nections should be taken to each item of 
apparatus and where several such items lie 
together, a subsidiary ring with short 
branches is preferable to a number of 
longer individual branches from the main 
bar. The aim should be to provide a mesh 
system wherever this can be contrived with 
reasonable economy. 

The operating mechanisms for outdoor 
airbreak switch disconnectors and earth 
switches and circuit breaker control kiosks, 
etc, not integral with the circuit breaker 
should be connected to the main earth 
grid by a branch earth connection entirely 
separate from that employed for earthing 
the air-break switch-disconnector or earth 
switch base, or the circuit-breaker struc- 
ture. The ftirther contribution to safety 
given by an insulated insert in the mech- 
anism drive is small compared with that 
obtained from such a branch earth connec- 
tion and, therefore, insulated inserts are not 
recommended in operating mechanisms of 
apparatus installed in substations. While sites 
covered with hard core and stone chippings 
will constitute a surface layer with a relati- 
vely high specific resistance, in the interests 
of safety, a metal grid can be provided at 
the operating points to give a level standing 
area and an earth connection made from 
this grid to the operating handle. 

Where it can be proved that the cur- 
rent carrying capacity of a main aluminium 
or steel member or welded sections forming 
a structure are at least equal to that of the 
required aluminium or copper earth con- 
ductor, the structure may form part of the 
connection and there is no need to fix an 
earth conductor along this section. A struc- 



ture made up of bolted sections should not 
be relied upon to form an efficient earth 
bond between equipment and the main 
earth grid, and loops bonding across 
structural joints are required. 

Connections to metal cladding, steel 
structure and metal door frames and win- 
dows or any other metallic panels should be 
made inside buildings. 

Where the earth wire of an incoming 
line ends at the terminal supports and is 
not connected to a point on the substation 
structures, a subsidiary earth connection 
should be provided between the substation 
earth system and the base of the support. 
If the latter lies outside the sub-station 
fence, the earth connection should be buried 
where it passes under the fence and should 
be kept well clear of the latter. 

Earth connections to surge diverters 
should be of sample cross-section and as 
direct as possible; they should not pass 
through iron pipes which would increase 
the impedance to surges of the connection. 
The earth connections of the diverters 
should be interconnected with the main 
earthing system since, for the effective 
protection of the substation equipment, a 
definite connection of low impedance 
between the equipment and the diverters is 
essential. 

20.6.2 Design 

20.6.2.0 General — The term earthing grid 
applies only to that part of the grid which is buried 
in soil. For design calculations of the grid resistance 
to the soil, only the buried part of the grid is to 
be taken into account. That part of the grid which 
lies embedded in concrete and also reinforcement 
connected to the grounding pads do lower the 
combined grid resistance but this contribution 
may not be taken into account while designing the 
earthing grid. 

20.6.2.1 Conductors installed above ground — 
Earthing conductors for power stations and sub- 
stations will normally be selected from copper or 
aluminium or steel sections adequately rated in size 
to carry the designed earth fault or three phase fault 
current for the appropriate designed maximum 
duration without exceeding a temperature given in 
Table 6A. Compliance with this requirement will 
additionally ensure satisfactory bonding without 
excessive voUage difference along any conductor. 

The required cross-sectional area of the earthing 
conductor is determined by the choice of condu- 
ctor material and the maximum duration of the 
fault current. The generally accepted duration for 
design purposes are one second for voltages above 
33 kV and 3 seconds for lower voltages. 



48 



IS : 3043 - 1987 



20.6,2.2 Conductors buried as strip electrodes — 
The earthing grid consists of the vertical pipe elec- 
trodes or plate electrodes interconnected by hori- 
zontal conductors which serve as a strip electrode 
(9.2.3) in addition to forming a earthing grid. It 
is recommended that the duration of earth fault 
current should be taken as one second for 230 and 
400 kV substations, and 3 seconds while designing 
earth grids for all other voltage levels. 

The other factors which shall be taken as the 
consideration while designing the earth grid are 
given below: 

a) Factor of safety for the ability of the earth 
conductor to carry the fault current during 
the period the fault persists, without any 
thermal and mechanical damage to the 
conductor; 

b) The relative importance of the installation 
for which the earthing system is being 
designed; 

c) The likely increase in the near future in the 
fault level in the area where the earth con- 
ductor has been installed; 

d) Operating time of the protective devices; 

e) Corrosion of the earth conductor; 

f) Factor of safety for workmanship in join- 
ting, etc; and 

g) Maximum permissible temperature raise for 
the buried part of the grid, which may be 
taken as 450°C for copper and steel condu- 
ctors. 

20.6.2.3 Sizing 

a) The cross-section of the area of the grid 
conductor shall not be less than the value 
stipulated in 12.2.2.1 where the value of ^ is 
to be taken as 80 for steel. This is based on 
a reasonable assumption that 3 seconds 
duration could not be adequate to bake 
out the ground moisture around the elect- 
rode especially as only a part of the current 
would be flowing across electrode-soil in- 
terface, 

b) A^t is a coefficient which takes into account 
the effect of number n spacing D, diameter 
<i and depth of burial /7 of the grid condu- 
ctors. 



JTt = 



_2 

2it 



/n 



D> 



+ 



1 



/„ 



Ibhd 

(i) (v) (-H 

Up ton-2 terms 

c) K^ is a coefficient which is similar to K^ 
dependent on the mesh width and the num- 



ber of parallel conductors given by the em- 
phirical relationship. 



K,^ -^ I, 



I 



2h 



+ 



I 



+ 



1 



'6D 



up to n terms 



ID 



+ 



e) Mesh potential 



(All lengths in metres) 

d) Kx is an irregularity factor to allow for non- 
uniformity of ground, dependent on the 
number of parallel conductors in the ground 
used in the mesh. 

K, - 0-65 4- 0-172 X n 

where n = number of parallel conductors. 

Mesh potential is the po- 
tential difference in volts 
from grid conductor to 
ground surface at centre of 
mesh grid. 

Mesh E « A't Ki F ~ 

where 

/ = fault current in amperes, and 
L = Length of buried conductor. 

f) The duration of fault for calculation of 
step, touch and mesh potential shall be 
the actual breaker fault clearing time. 

20.6.3 Construction 

20.6.3.1 General — It is essential for the 
safety of personnel and plant that an earth system 
should remain effective throughout the lite of the 
plant. It is difficult in many cases to make a check 
of continuity after installation. The system, there- 
fore, has to be robust and protected from mech- 
anical damage and corrosion, where necessary. 
Any joints should be capable of retaining low resis- 
tance after many passages of fault current. 

20.6.3.2 Laving conductors — Buried bare 
copper or steel conductors forming part of the ear- 
thing system should be at about 600 mm deep 
which, in addition to giving protection to the con- 
ductor and connections, should ensure that it will 
normally be below frost line. Aluminimum should 
only be used for above ground connections. 

NOTE — If the indigeneous soil is hostile tO copper, 
that is, acidic with a pH value of less than 6 or alkaline 
with a pH value of more than 10, suitable surrounding 
soil should be imported. 

Where an adequate earthing installation is pro- 
vided, the subsidiary connections from the main 
earth grid to equipment may be laid at a depth 
and by routes most appropriate to site connections. 
For convenience in connecting to equipment, they 
may be laid at a depth of about 250 mm, and 



49 



IS: 3043-1987 



as they are, therefore, in ground more subject to 
seasonal or progressive changes of resistivity, it 
may be assumed that they make negligible con- 
tribution towards reducing station earth resistance. 
On the other hand, they do serve to reduce surface 
gradient within the station site. Conversely where 
these connection are also required to improve the 
earth value of the station, the 600 mm depth is 
required. The above recommendations deal mainly 
with stations on normal sites. Where ground con- 
ditions restrict the installation depth or where the 
soil resistivity is excessive, additional measures 
may be required beyond the station boundary to 
improve the overall earth value. 

The earthing installation within the station 
will, however, bond the station plant and restrict 
touch potentials to acceptable limits. 

Where bare metal conductor is buried under 
metal fencing, and the fencing is independently 
earthed, the conductor should be insulated by 
threading through non metallic pipe extending for 
at least 2 m each side of the fence or alternatively 
insulated conductor may be used. 

When laying stranded conductor for earthing 
purposes, care should be taken to avoid birdcaging 
of the strands. 

20.6.3.3 Fixing conductors — In fixing alu- 
minium or copper conductors to structures, etc, 
insulated clips should be used to avoid drilling and 
prevent electrolytic action. Galvanized clips should 
not be used. Fixing should be spaced not more 
than 1 m apart. 

Earth conductors in trenches containing power 
and/or multi-core cables should be fixed to the 
walls near the top (for example, 100 mm from 
the top). 

Copper earth strip supported from or in con- 
tact with galvanized steel should be tinned to pre- 
vent electrolytic action. 

Sharp bends required in aluminium strip should 
be formed by the use of a bending machine. 

Aluminium earthing conductors will give satis- 
factory performance in contact with concrete, 
cement, plaster and brickwork, and may be buried 
in concrete or plaster, provided it remains dry 
after setting. In outdoor installations, the conduc- 
tor will weather to a grey appearance and in 
marine or industrial atmospheres slight surface pit- 
ting may occur. This will not aftect performance 
since the sections are relatively large. The inter- 
faces of all 'mechanical-joints should be protected 
with a suitable electrical joint compound, parti- 
cularly any bimetallic joints. All bimetallicjoints 
should then be encapsulated in a grease impre- 
gnated tape, mastic compound or bitumastic paint, 
etc, to exclude moisture. 

In general, aluminium should only be used 
above ground and the connections to earth ele- 



ctrodes made above ground with bimetallicjoints. 
Aluminium can be used below ground only if effi- 
ciently protected or sheathed against contact with 
soil and moisture. 

20.6.3.4 Jointing conductors 

a) General — All crossings of conductors in the 
main earth grid should be jointed. Com- 
pression type joints may be used for stran- 
ded conductors. Non-conductor strip should 
be drilled for a bolt having a diameter 
greater than one-third of the width of the 
strip. If this diameter will be exceeded, than 
a wider flag should be jointed to the strip. 

b) Aluminium to aluminium — When possible, 
joints on strip conductor should be arc wel- 
ded using either the tungsten inert-gas arc 
(TIC) or metal inert gas arc ( M I G ) 
techniques. Oxy-acetylene gas welding or 
brazing may also be used. 

Ranges of compression fittings and tools 
are available for round conductors. Round 
conductors can also be flattened and 
punched with suitable tools to form a ter- 
minal. 

Round and rectangular conductors can 
bejoined with bolted clamps. 

Rectangular conductors can bejoined 
or terminated by drilling and bolting. 
When making a bolted type joint, the 
surface of the aluminium should be cleaned 
thoroughly by wire brushing and greased 
or an approved jointing compound applied 
immediately to both mating surfaces. 
Bolts should then be tightened and all 
excess grease or compound wiped otTand 
discarded. 

To ensure adequate contact pressure 
and avoid overstressing, torque spanners 
should be used. The conductor manufac- 
turers literature should be consulted for 
further details for the joints and proced- 
ures. 

Cold pressure welding and explosive 
bonding can be used for jointing rectang- 
ular conductors. The appropriate manu- 
facturer should be consulted for details of 
these procedures. 

c) Aluminium to copper — Joints between alu- 
minium and copper should be of the bolted 
type and be installed in the vertical plane 
at a minimum distance of 150 mm above 
ground level. 

The rating surface of the aluminium 
should be cleaned thoroughly by wire brush- 
ing and greased or an approved jointing 
compound applied and the copper tinned. 
Grease or an approved jointing compound 
should be applied to the melting surface of 



50 



IS : 3043 - 1987 



the aluminium. After bolt tightening by 
torque spanner, excess grease or com- 
pound should be wiped off and discarded, 
and the joint protected from the ingrease 
of moisture by the application of suitable 
plastics compound or irradiated polyethy- 
lene sleeve with mastic lining. Alternatively, 
the joint may be protected by a bitumastic 
paint. 

Aluminium conductor connections to 
equipment should, where possible, be in the 
vertical plane. Surface preparation of the 
aluminium and the making of the joint 
should be as previously described. The 
finished joint should be protected by a 
bitumastic paint. 

d) Copper to cqjper — The following methods 
may be used: 

1) Brazing using zinc-free brazing material 
with a melting point of at least 600°C; 

2) Bolting; 

3) Riveting and sweating; and 

4) Explosive welding. 

Earthing conductor connections to equip- 
ment should, as far as practicable, be 
made onto vertical surfaces only. In the 
case of painted metal, the paint should be 
carefully removed. Earthing conductors 
should be tinned where connected to 
galvanized steelwork. No connection point 
should be less than 150 mm above ground 
level. In any position, subject to corrosion, 
the finished joint should be protected by 
bitumastic paint. 

e) Loops for portable earths — Loops of plain 
aluminium or copper should be provided 
on the earth conductor at each location 
where portable earthing leads may be app- 
lied. The loops should not be less than 180 
mm long and 75 mm clear of the earth con- 
ductor; they should be at a convenient 
height and should be formed separately, 
not by bending the earth strip itself. Loops 
should be jointed to the earth conductor 
using a method given in 20.6.8.4 (d). 

f) Steel — For steel, it is recommended to use 
only welded joints. 

20.7 Earthing of High Voltage Cable Sheaths 

20.7, 1 Three-Core Cables — Modern high voltage 
power cables are generally provided with a poly- 
meric insulating oversheaths. The sheath of solid 
tpye cables are generally directly earthed at their 
terminations and joints, the cable sheaths being 
bonded at joints. The sheath earth connections 
of pressure type cables are generally made via a 
removable link in a lockable box to permit 
periodic testing of the oversheath insulation, the 
joints being insulated, but the sheaths bonded 
through. The test requirement also means that 
insulating glands should be provided at the cable 



termination boxes of transformers, switchgear, etc 
and at cable sealing ends or joints. 

20.7.2 Single-Core Cable Tails — The sheaths of 
single-core cables have a longitudinal induced 
voltage, the magnitude of which is directly pro- 
portional to the current flowing in the core. When 
both ends of a single-core cable are bonded to 
earth, a current flows in the sheath and the ther- 
mal effects of this sheath current derates the 
capacity of the cable core. Where this derating is 
unacceptable and the value of the standing 
induced voltage is acceptable, it is usual to 
earth the sheaths of the single-core cables at the 
trifurcating box or in the case of single-core mains, 
the end of the trefoil formation, the cable glands at 
sealing ends or plant cable boxes being of the in- 
sulated type. The acceptable level of the maximum 
sheath voltage is generally taken as 65V with flill 
rated current flowing in the cable, but where the 
ratio of fault current to full rated current is 
so high that the voltage developed across an insu- 
lated gland is unacceptable, it is necessary to der- 
ate the permissible voltage to some level lower 
than 65 V. 

20.7.3 Single-Core Cable Mains — The choice of 
termination and earthing arrangements for single- 
core cable mains is a matter of economics. The 
possible methods of earthing are as follows: 

a) Solid Bonding — In this system, the sheath 
bonding and earthing arrangements are 
such that the sheaths are maintained near 
earth potential throughout their length. 

b) Single Point Bonding — This method is as 
described in 20.7.2 for single core tails, and 
is subjected to practical limitations of cable 
lengths permissible. 

c) Cross-Bonding — In this method, the cable 
length is divided into three equal sections 
(or into a multiple ofthree such sections) 
and at each section junction, an insulating 
joint is provided. At these joints, the sheath 
of each cable section is bonded to the she- 
ath of a different phase cable of the next 
section through lockable link boxes. By suit- 
able connection, the phaser sum of the 
longitudinal sheath voltage is zero, and at 
the cable terminations, the sheaths of all 
three cables are bonded to earth. It is usual 
to provide a three-phase star-connected set 
of cable protections at each intermediate in- 
sulatingjoint; these protectors are non-linear 
resistors presenting low impedance to surge 
currents. The cross-bonding method permits 
the Ml rating of the cable to be maintained, 
but incurs considerable cost in the provision 
of insulating joints, link boxes, protectors, 
etc. 

20.8 Miscellaneous Matters in Power 
Stations and Substations — If two or more 
stations are adjacent on what may be considered 
to be one site, the earthing systems and the stations 
should be interconnected to form a single earthing 



51 



IS: 3043-1987 



system. Where the stations actually adjoin, the 
extremities of their earthing systems should be 
connected together so that the whole area is en- 
closed by the earthing system. Where the separ- 
ation is too large to treat as adjoining stations, an 
interconnecting earth conductor of substantial 
cross-section should be run to ensure that, as far 
as practicable, fault currents are diverted from 
cable sheaths and armour. This is of particular 
imporatnce where fault current flowing in one 
station is provided from the adjoining station, for 
example, where a switching station adjoins power 
or transforming station sites so that an earth fault 
in the switchgear causes current flow between the 
two sites in order to reach the system neutral at 
the generators or transformers. Such interconne- 
ctions between sites can include links suitably dis- 
posed to assist in testing. 

Except where special insulation is called for, 
sheaths of all main cables should be connected to 
the station earth system. With multi-core cables 
the connection is generally made at the termina- 
tion. 

Where high earth-fault currents are to be ex- 
pected, and an appreciable rise of potential of the 
station system with respect to the general body of 
the earth may ensure, special care is necessary 
with connections other than main cables or lines 
entering the station, such as water pipes and tele- 
phone or pilot cables, water pipes should include 
an insulated section; polymeric piping is often 
suitable. In several cases, isolating transformers 
may be necessary for telephone connections. Bri- 
tish Telecom provides isolation equipment at their 
circuit terminations when the potential rise exceeds 
430 V (650 V for high reliability lines). Pilot 
cables should be provided with insulated glands 
and so disposed as to minimize the possibility of 
fault currents being carried by the sheaths. 

Where carrier-current equipment is employed, 
a further earth-electrode, normally a driven rod, 
should be provided at or immediately adjacent to 
each structure supporting the coupling capacitors. 
This earth electrode is an additional one for the 
high frequency equipment and should be bonded 
into the main earthing system. The structures 
supporting the coupling capacitors should be ear- 
thed in the normal way. 



EARTHING ASSOCIATED 

OVERHEAD POWER LINES 



WITH 



21.1 Type of Support — Any consideration of 
whether metalwork associated with overhead 
power lines should be earthed and/or bonded has 
to take account of the type of support. Some over- 
head lines are supported by lattica towers of meta- 
llic construction, others by poles, which may be 
of steel, wood, concrete or of fabricated cons- 
truction, for example, glass-reinforced plastics; 
brackets attached to buildings are also used to 
support conductors. 



21.2 Insulation Failure — Following an insula- 
tion failure, a voltage may exist between any sup- 
porting metalwork and earth. The public are 
generally protected if no metalwork within 3 m of 
the ground is liable to become live on failure of 
insulation. If the supports are close to buildings, 
etc, the particular circumstances have to be 
considered. 

21.3 Lattice Steei Structures — There will 
often be satisfactory earthing of lattice steel struc- 
tures, poles of metallic construction and reinforced 
concrete poles through their contact with the 
ground. In areas of high earth resistivity, special 
earthing arrangements may be necessary; an over- 
head protective conductor attached at each support 
and connected to the neutral of the supply and of 
the line may be the most economical solution. 
This conductor if positioned above the live con- 
ductors, will also provide a measure of lightning 
protection. 

21.4 Poles of Non-conducting Material 

21.4.1 General — Where a pole is of non-con- 
ducting material, for example wood or glass-rein- 
forced plastics, the pole will act against the flow 
of leakage current and can be expected to prevent 
danger near ground level due to leakage across or 
failure of any insulator supporting a line conductor, 
except where there is intervening equipment or 
metalwork that is or may become live. 

For the reasons given in 21.4.2 to 21.4.5, there 
are advantages in not earthing the pole-top metal- 
work of such poles and in not making bonding 
connections to it. 

21.4.2 Omission of Bonding — Where insulators 
are attached to a pole or to non-conducting cross- 
arms, etc, attached to the pole, ommission of 
bonding of pole-top metalwork gives a greater 
impulse withstand voltage, so there is less risk of 
faults due to phase-to-phase flashover. To reduce 
risk of fire, where wooden cross-arms are used, 
care should be taken to make close, fire contact 
between the cross-arm and the insulator pipe. 

21.4.3 Omission of Earthing — If pole top metal- 
work is not earthed, transient faults due to birds, 
flying branches, etc, bridging the clearance 
between line conductors and the metalwork are 
greatly reduced. 

21.4.4 Transformers, Rod-operated Switchgear and 
Cable Terminations — In cases where equipment, 
such as transformers, rod-operated switchgear or 
cable terminations are mounted on a wooden or 
reinforced plastics pole, the impulse flashover 
value of the additional insulation provided by the 
pole is impaired, and all the metal work on the 
pole needs to be bonded and earthed. 

21.5 Stays — To prevent stay corrosion that would 
otherwise occur due to passage of small leakage 
currents occurring even in normal operation, stay 
insulators should be fitted in stay wires on poles. 



52 



IS : 3043 . 1987 



No part of the stay insulator should be less than 
3 m above ground; it should be fitted as high up 
the stay as possible, but the stay insulator should 
be so positioned that there can be no contact below 
the stay insulator between the stay wire and any 
phase conductor (including a jumper connection), 
should either of them break or become loose 

21.6 Metal Brackets Attached to Buildings — 

A metal bracket attached to or adjacent to any 
metalwork on or joining part of any building or 
structure and supporting a phase conductor needs 
to be earthed unless the conductor is both insula- 
ted and supported by an insulator, each form of 
insulation being suitable for the conditions under 
which it will be required to operate in the event 
of failure of the other. 

21.7 Earth Wires and Earth Connection — 

Any connection between metalwork and earth has 
to be of low resistivity, both to provide for prompt 
operation of protective equipment and to minimize 
inductive interference with communications circuits 
in the event of a flow of fault current. Electro- 



magnetic interference is reduced if the resistance 
of the earth return path is small compared with its 
reactance. At 50 Hz, inductive interference rnay 
be caused by the use of a high-resistivity wire (for 
example, steel wire) even if it is perfectly earthed. 
A single low-resistivity earth wire made of copper, 
aluminium etc, should be used and it should avoid 
passing close to conductors or cables belonging to 
other circuits. It should be protected against 
mechancial damage for a distance of 3 m above 
ground level. 

21.8 Lightning Protection — A lightning con- 
ductor attached to a structure and earthed at its 
lower end acts to reduce the likelihood of a lightn- 
ing strike. An over-running aerial earth-wire on 
overhead power line, besides forming part, of the 
earth return path, also gives a degree of lightning 
protection. The lower the impedance between 
aerial earth-wire and earth, the better is the 
protection since this reduces the possibility of a 
back flashover from the earthed metalwork to line 
conductors on the occasion of a direct strike to the 
earth wire. 



SECTION 5 INDUSTRIAL PREMISES 



22. GUIDELINES ON EARTHING 
INDUSTRIAL PREMISES 



OF 



22.1 Genera! — The design of earthing system 
for any scheme is developed on the basis of basic 
requirements. 

22.1.1 So far as the consumers taking supply 
at 240 V are concerned according to the provi- 
sions of the basic statutes, it is the responsibility 
of the supplier to provide earthed terminal at the 
premises of the consumer. In the cases of consu- 
mers taking supply at higher voltages, earthing 
scheme should be so designed as to satisfy the 
basic statutory requirements and also to provide 
adequate protection against ground faults. 

22.1.2 The earthing system in the premises of 
consumers at voltages above 240 V should be 
designed as a PMP system with separate protec- 
tive conductor. The neutral of the transformer 
should be connected to be earth electrodes by 
duplicate connections and adequate number of 
earth electrodes should be provided with inter- 
linking earth bus for getting an optimum value of 
the earth resistance depending upon the setting of 
the earth fault/earth leakage relays and also to 
limit the extent of rise of potential in the case of 
solidly earthed system, the ground fault current 
can be of the order of symmetrical short-circuit 
current and hence the thermal design of the 
earth bus and the earthing system should depend 
upon the maximum symmetrical short circuit 
current available. The duration of the earth fault 
current according to the existing design practice 
is 3 seconds. However, in case of installations 
where adequate protective arrangements have 



been incorporated so as to instantaneously isolate 
the system in the event of a ground fault, a lesser 
duration can be considered for design purposes. 

22.1.3 As far as the value of the earth resis- 
tance is concerned, the objective from the point 
of safety consideration is not to attain minimum 
value of the earth resistance as is sometimes 
understood. But the consideration should be 
whether there is adequate co-ordination between 
the practically obtainable value of the earth 
resistance and setting of the protective relays. 
This aspect is very much relevant in the case of 
installations where the value of the earth resisti- 
vity which is to be taken for the calculations is 
abnormally high. The disposition of the earth 
electrodes, and the extent and size of earth grid 
will always depend upon the disposition of plant 
electrics; the layout should be done in such a 
manner as to keep the earth continuity resistance 
to within the stipulated figure. The thermal 
rating of the earth electrode is specified by this 
code which gives the formula for the maximum 
allowable current density in an earth electrode. 
However, in the case of a protective multiple 
earthing system where the neutral of the supply 
transformer and the non-current carrying metal 
parts in the system are interconnected by the 
common earth grid, which is designed for the 
prospective fault current, there is no reason to 
design the earth electrodes assuming that total 
earth fault current is dissipated through the earth 
electrodes. In the case of an interconnected 
system, earth fault current is returned to the 
neutral mostly through the interconnected system, 
earth fault grid. However, depending upon the 



53 



IS : 3043 - 1987 



value of the earth resistivity, a percentage of the 
current may flow through the mass of the earth 
as well The current, which takes the earth return 
path, enters the earth through different earth 
electrodes. Flence, while designing the earth 
electrodes, the thermal capability of the earth 
electrodes need be verified only with reference to 
the portion of the current which may take the 
earth return path, which depends upon the earth 
resistivity. In the normal range of earth resistivi- 
ties between 10 and 1 000 m, this division of 
current is found to be in between 80 percent and 
20 percent for design purposes. Hence, depending 
upon the disposition of the plant electrics, an 
optimum number of earth electrodes are provided 
as anchorages for the earth grid. The value of 
the earth resistance of the grid so formed is then 
calculated assuming the bare interconnected grid 
as a strip electrode. The value of the earth 
resistance so obtained should be within reason- 
able limits, otherwise brought down by adding 
more etectrodes. The ground fault protective 
device or the phase fault protective device (in 
case there is no ground fault protective device) 
is set to operate at the minimum current which is 
obtainable under a ground fault condition. The 
thermal rating of the earth electrodes are then 
cross verified, based upon the percentage of 
current which takes the earth return path. Based 
upon the above philosophy, the following guide- 
lines for the design of an earthing system in the 
HT consumers premises are issued. 

22.2 Consideration for Earthing 

22.2.1 The main earthing conductor will be 
run in between standard earth electrodes 
conforming to specifications and distributed 
uniformally around the working area. All the 
non-current carrying metal parts of the equip- 
ments, switchboards, etc, will be soUdly connected 
to this earth grid or equipotential bonding con- 
ductor by duplicate earth connections of adequate 
size. For interconnecting switchboards protected 
by HRC fuses to this earth grid, the size of 
interconnection need not be more than 75 mm" 
copper or its equivalent. In laying out the earth 
electrodes and the earth conductors, all efforts 
should be made to maintain a uniform potential 
gradient in and around the work area. The 
transformer neutral should be solidly connected to 
this grid by duplicate earth connections, one going 
directly to earth electrodes and other going to the 
common earth bus. The size of the neutral earth- 
ing conductor should in no case be less than that 
of the size of the main earthing conductor. 

22.2.2 The earth grid should be run at a mini- 
mum depth of 50 cm below ground. When bare 
conductors are used as earth grid, this can also be 
assumed to dissipate the fault current to the mass 
of the earth and for calculating the effective value 
of the earth resistance of this grid, this grid can 
be treated as a strip electrode and the standard 
formula can be applied for calculating the earth 
resistance of the grid. 



22.2.3 The continuity resistcince of the earth 
return path through the earth grid should be 
maintained as low as possible and in no case 
greater than one ohm. 

22.2.4 In the case of EHT substations, where 
there is possibihty of the ground potential attain- 
ing very high values (of the order of 5 kV and 
above) in the event of an earth fault, the earth 
grid design should be based on the tolerable limits 
of the potential gradient in the substation area, 
and the step and touch potential due to fault 
conditions. 

22.2.5 In the case of EHT substations, the 
earth conductors should be bare and they should 
be buried direct in ground. 

22.3 The Earth Electrodes 

22.3.1 The earth electrodes are provided to 
dissipate the fault current in case of earth faults 
and to maintain the earth resistance to a reasona- 
ble value so as to avoid rise of potential of the 
earthing grid. Practice, which has been followed 
uptil now, is to design the earth electrodes for 
the appropriate thermal withstand capacity, 
assuming the total fault current to be passing 
through the earth electrodes. This is true in the 
case of an earthing system which is not inter- 
connected with neutral earthing (TT/IT system). 
But with the adoption of PME system in industrial 
distribution where the neutral is solidly connected 
to the earthing grid, the above practice requires 
revision as has already been pointed out in 22.1.3 
in order to avoid redundancy and thereby to 
avoid unnecessary expenditure. The amount of 
current that may actually be dissipated through 
the earth electrodes depends to a large extent, on 
the earth resistivity of the soil. Depending upon the 
value of the earth resistivity, the total fault current 
from the supply system will return to neutral 
partially through the earth grid and partially 
through the earth return path. The percentage of 
current which flows directly through the earth 
grid depends on the resistance of the earth return 
path in relation to the earth resistivity. The 
standard earth resistivity values typically vary in 
the range between 10 and 1 000 ohms. In this 
range of variation, it can be reasonably assumed 
that the fault current division at the point of 
entry to the earth grid is 20 to 80 percent. For 
verification of the fault dissipating capacity of 
earth electrodes, only the portion of the fault 
current which is diverted to the earth electrode 
need be taken and under these conditions the 
maximum allowable current density as stipulated 
in this code should not be exceeded. 

22.3.2 The number of earth electrodes required 
for a particular installation will be basically deci- 
ded by the optimum value of the earth resistance 
which is required to make the protective system 
operation. Hence, the optimum value of the earth 
resistance depends upon the reasonable potential 
rise and setting of the earth fault isolating devices 



54 



IS : 3043 - 1987 



or the series, protective devices in case where there 
is no ground fault detecting devices. The main 
criterion is that the value of the earth return 
resistance should not be so high as not to produce 
the required ground fault current for actuating 
the protective devices within the stipulated time. 
Or in other words, the optimum value of the 
earth resistance is closely related to setting of the 
earth fault protective devices used in the system. 
For a small installation, as a general rule, in the 
event of a direct earth fault the earth fault cur- 
rent produced should not be less than five times 
the highest rating of the maximum protective 
tlises or the setting of the earth fault relay if 
such a device is provided. 

22.4 Determination of Earth Resistivity — 

As has already been pointed out, the value of the 
earth resistivity plays an important role in the 
design of the earth electrodes. In the conven- 
tional method, the earth resistivity which is to be 
applied in the design calculations is taken as the 
arithmetic mean of a number of measured values 
in the area under consideration. The figure so 
obtained seldom projects a realistic value. A more 
scientific approach is to measure the earth resis- 
tivity in different radial directions from a central 
point which may be taken as the proposed load 
centre. With the values so obtained, a polar 
curve is drawn. The polar curve is converted to 
an equivalent circle (see 36,6). The radius of 
the circle is taken to be the average value of the 
earth resistivity figure which is to be applied in 
design calculations. Necessary allowance should, 
of course, be given for factors such as variations 
in climatic conditions, treatment of soil, etc. 

22.5 Design ofEarth Bus 

22.5.1 Design of earth bus is based upon the 
general guidelines given in Section 2. The size of 
the main earth grid will be decided on the basis 
of line to ground fault current assumed to be 
symmetrical short-circuit current in the system. 
This assumption is fairly reasonable in the case of 
a solidly earthed system where the ratio between 
XO/XI is limited to less than 3 and the ohmic 
value of the earth return path to the supply neutral 
is reasonably low. The minimum iault level 
existing at the supply point will be assumed to be 

13.1 kA or the actual fault current whichever is 
greater for premises at voltages above 1 kV. 

22.5.2 Bare copper, PVC covered aluminium 
or Gi subject to relevant restrictions based on 
the location and nature of installation may be 
used as earthing to conductors. The size of the 
earthing conductors will be calculated according 
to guidelines given in the code. The time dura- 
tion of the fault current as recommended is 3 
seconds. According to standards developed in this 
regard, the size of the earthing conductors will 
be based upon current densities as given in 
Section 2 of this code. A corrosion factor of 5 
percent of unit drop in the value of corrosion 



index up to — 10 is recommeded for steel/GI 
earthing conductors while designing an earthing 
scheme, situations of corrosion index of below 
— 10 should not be allowed. 

22.5,3 In the case of systems where standard 
protective arrangments have been provided for 
isolating the ground txjults instantaneously, due 
consideration can be given to this aspect in decid- 
ing upon the size of the earthing conductor by 
giving due allowance to lower duration of the 
ground fault currents. 

22.6 Correlation Between Grounding and 
Earth Fault Protection 

22.6.1 The phase f^ult protective device 
normally used in systems operating at 415 V 
afford reasonable protection against arcing ground 
faults. The ground fault current depends upon 
the impedance to zero sequence current flows 
and depends to a large extent on the grounding 
network and the earth resistivity. The pick up 
value of the ground fault relays or the value of 
the phase fault protective device should be co- 
ordinated for the required protection for the 
system. In case the impedance of the earth return 
path for ground fault current cannot be regulated 
so as to produce adequte fault current for operat- 
ing the phase fault protective devices like fiases, 
such circuits should be protected by separate 
ground fault protective devices. Hence, the nece- 
ssity of separate ground fault protection depends 
on the grounding network and its effective impe- 
dance and earth grid design is closely related to 
the effectiveness of the phase fault protective 
device in clearing a ground fault in place where 
separate ground fault protective devices are not 
provided. 

22.7 Grounding and Ground Fault Pro- 
tection 

22.7.1 In recent, years, there has been an 
increasing interest in the use of ground t^ult 
protection in industrial distribution circuits. This 
interest has been brought about by a disturbing 
number of electric failures. Hence it is worthwhile 
to explore the need for better ground fault pro- 
tection and to examine the grounding practices in 
the light of the required protection. 

22.7.2 Distribution circuits which are solidly 
grounded or grounded through low impedances 
require fast clearing of ground faults. This in- 
volves high sensitivity in detecting low groud fault 
currents as wall as the co-ordination between 
main and feeder circuit protective devices. Fault 
clearing must be extremely fast where arcing is 
present. 

22.7.3 The appeal of effective ground fault 
protection is based on the following: 

1) The majority of electric faults involve 
ground. Ungrounded systems are also sub- 



55 



IS : 3043 - 1987 



ject to ground faults and require careful 
attention to ground fault detection and 
ground fault protections. 

2) The ground fault protective sensitivity can 
be relatively independent of continuous 
load current values and thereby have 
lower pick up settings than phase protec- 
tive devices. 

3) Ground fault currents are not transferred 
through system, in the case of power trans- 
formers which are connected delta-star, 
delta-delta. The ground fault protection 
for each system voltage level should be 
independent of the protection at other 
voltage levels. This permits much faster 
relaying than can be afforded by phase 
protective device which require co-ordinate 
using pick up values and time delays which 
extend from the load to the service genera- 
tors, often resulting in considerable time 
delay at some parts in the system. 

4) Arcing ground faults which are not prom- 
ptly detected and cleared can be extremely 
destructive. A relatively small investment 
can provide very valuable protections. 

22.8 Much of the present emphasis on ground 
fault protection centres around or circuits below 
550 V. Protective devices have usually fuse 
switches of circuit breakers with integrally moun- 
ted phase tripping devices. These protective 
elements are termed as overload or fault overcurr- 
ent devices because they carry the current in each 
phase and clear the circuit only when the current 
reaches a magnitude greater than full load 
current. To accommodate inrush currents such as 
motor starting or transformer magnetising inrush, 
phase over current devices are designed with 
inverse characteristics, which are rather slow at 
overcurrent values upto about 5 times rating. For 



example, a 1 600 A circuit breaker with conven- 
tional phase protection will clear a 3 200 A fault 
in about 100 seconds. Although it can be adjusted 
in the range of 30 to 200 seconds, at this fault 
value. A I 600 A fuse may require 10 minutes or 
more to clear the same 3 200 A fault. These low 
values of fault currents are associated predomi- 
nantly with fault to ground and have generally 
received little attention in the design of earthing 
systems, until the occurrence of many serious 
electric failures in recent years. In contrast, on 
grounded systems of 3 3 kV and above, it has 
been a standard practice to apply some form of 
ground fault protection. 

22.9 The action initiated by ground fault sensing 
devices will vary depending upon the installa- 
tion. In some cases, such as services to dwelling, 
it may be necessary to immeditately disconnect 
the faulted circuit to prevent loss of life and pro- 
perty. However, the opening of some circuits in 
critical applications may in itself, endanger life 
or property. Therefore, each particular applica- 
tion should be studied carefully before selecting 
the action to be initiated by the ground fault 
protective devices. 

22.10 Protection Against Arcing Ground 
Faults and Earth Leakage 

22.10.1 Necessity of arcing ground fault protec- 
tion especially for 415 V installations is not very 
well understood and protective schemes suggested 
for normal industrial installations never give 
much importance to this aspect. It is also seen 
that the fact that a series protective device like 
breaker or a fuse does not offer protection against 
an earth fault or arcing ground fault in a 415 V 
system, is very often forgotten. In the case of 
such installations, the avoidance o\^ arcing ground 
faults is important from the point of view of per- 
sonal safely and equipment damage. 



SECTION 6 STANDBY AND OTHER PRIVATE GENERATING PLANTS 



23. EARTHING IN STANDBY AND OTHER 
PRIVATE GENERATING PLANTS (IN- 
CLUDING PORTABLE AND MOBILE 
GENERATORS) 

23.1 General — The earthing of standby and 
other private generating plant is necessary to 
protect against indirect contact that may result in 
electric shock. The objective is to create a zone 
in which voltage between exposed conductive 
parts and extraneous conductive parts are minimi- 
zed in the event of an earth fault. 

In this section the requirement is met by con- 
necting the generating set frame(s), metallic cable 
sheaths and armouring, and all exposed conduc- 
tive parts to an earthing conductor, and by 
connecting the system to earth (normally at one 
point only). 



Except in some special applications, there is, 
in every case, need for an independent earth 
electrode for energy source earthing at the pre- 
mises where the generator is located. (Any 
suppliers' protective earth terminal at the premi- 
ses should also be connected to the independent 
earth electrode). 

There are many variations in system design and 
for any particular application, the precise method 
of energy source earthing is subject to the recom- 
mendations of the machine manufacturers, the 
system parameters and, where mains supplies are 
also involved, the agreement of the concerned 
supply authority. 

It may, however, be noted that the guidance 
included in this section, applies to stock protec- 
tion as well as protection of equipment. 



56 



IS : 3043 - 1987 



23.2 Low Voltage Up to 1 000 V Generators 

23.2.1 Earth Electrodes — The overall resistance 
to earth of the electrodes forming the connection 
to the general mass of earth from the low voltage 
energy source has to be consistent with the earth 
fault protection provided and shall be as low as 
possible. 

23.2.2 Single Low Voltage Generator Earthing 
{Synchronous Machines) 

23.2.2.1 Generator operating in isolation {from 
the mains or other supplies) — In this basic arrange- 
ment, the generator neutral point should be 
connected to the neutral of the low voltage 
switchgear which is itself connected through a 
bolted link (for test purposes) to an earthing 
conductor and the independent earth electrode. 

23.2.2.2 Standby generator {without paralleling 
facility) — In addition to the earthing require- 
ments stated for a set operating in isolation from 
other supplies, special attention needs to be given 
to the change-over arrangement for standby set, 
which has to ensure that there can be no inadver- 
tent parallel connection {see Fig. 24). 



In general four-pole changeover switching 
between the mains and standby, supplies should 
be used to provide isolation of the generator and 
electricity board neutral earths. However, in the 
case of a protective multiple earthing (PME) 
supply, three- or four-pole switching may be used. 

23.2,2.3 Standby generator {capable of parallel 
operation with incoming wains supply) — Electricity 
boards will not generally permit continuous parallel 
operation of a synchronous machine with the low 
voltage mains supply, unless there are no other 
consumers on the network. However, short-term 
parallel operation for no-break load transfer or 
testing may be permitted. Also, if a synchronus 
machine output is rectified and connected through 
a mains modulated static inverter continuous 
parallel operation will usually be permitted. In 
the latter case, the generator neutral terminal 
should be connected to the independent earth 
electrode and to any electricity board earth. 

For short-term parallel operation, giving no- 
break load transfer, the alternative energy source 
earthing arrangements, which may be used, are 
as described in 23.2.3,1, except that only one 
generating set is involved. 



PRIME 
MOVER 



CHANGEOVER FACILITY 
(AUTOMATIC OR MANUAL) 



LRATOR 



INCOMING 
Rk ELEC BOARD 
Y 



EXTRANEOUS 

CONDUCTIVE 

PARTS 




INDEPENDENT EARTH ELECTRODE 



EARTHING BAR 



NOTE I — Cable sheath earth of provided/shown - 
NOTE 2 — PNE link of provided/shown 



r»v. 



L»fJ 

NOTE 3 — Changeover switch could be 3-pole with linked neutral. 

FIG. 24 SINGLE Low VOLTAGE STANDBY GENERATOR (WITHOUT PARELLELING FACILITY) 



57 



IS : 3043 - 1987 



23.2.3 Multiple Low Voltage Generator Earthing 
{Synchronous Machines) 

23.2.3.1 Generator operating in isolation from 
other supplies — When low voltage generating sets 
are operated in parallel, the energy source earth- 
ing method is influenced by the magnitude of the 
circulating currents, particularly third harmonic, 
which can arise when generators are connected 
as four-wire machines. If the magnitude of the 
circulating current due to the nature of the load 
or the design of the generators is excessive when 
the neutrals are connected, then a neutral earth- 
ing trnsformer or star-point earthing switches are 
required. 

Hence, three alternative neutral earthing 
arrangements are possible for parallel operation 
as follows: 

a) All generator neutrals connected — With this 

arrangement, the neutral busbar in the 
main low voltage switchgear is connected 
through a bolted link to an earthing con- 
ductor and independent earth electrode. 

b) Neutral earthing transfonver — By providing 

a neutral earthing transformer solidly con- 
nected to the busbars, the system neutral 
can remain earthed at all times whilst any 
number of generators can be connected to 
the busbars as three-wire machines. 



c) Generator star point switching — When this 
arrangement is adopted, it is necessary 
before the first generator is started for its 
star-point/neutral earthing switch to be 
closed. When subsequent sets are started, 
their star-point earthing switches remain 
open. This avoids the circulating current 
problem, but it is essential that electrical 
and mechanical interlocks on the star- 
point/earth switches ensure the integrity of 
the energy source neutral earth connection 
at all times and under all possible operating 
conditions. 

23.2.3,2 Standby generators (without mains 
paralleling facility) — The alternative neutral 
earthing arrangements for standby generators are 
as set out in 23.2.3.1 for generators operated in 
isolation from an electricity board supply. The 
earthing arrangements are shown in the following 
drawings: 

a) All generator neutrals connected {see 
Fig. 25); 

b) Neutral earthing transformer {see Fig. 26); 
and 

c) Alternator star-point switching (Fig. 27). 

For standby generators with no mains para- 
lleling facility, the changeover arrangement has 
to prevent inadvertent connection of the genera- 
tor outputs and electricity board supply. 



PRIME 

Mover 



R Y B N 



EXTRA NEOU 

CONDUC 

PARTS 




INCQMtNG 

wkElec board 

'TTyv SUPPLY 



EOuTl-. -J- -.---, 



!N0€ 
EARTH EUECTROOE 



NOTE I — Cable sheath of provided/shown — 



NOTE 2 — PNE link of provided/shown 



1*1 



NOTE 3 — Changeover switch could be 3-pole with linked neutral. 

FIG. 25 Low VOLTAGE STANDBY GENERATORS WITH NEUTRALS CONNECIED 



58 



IS : 3043 - 1987 



NEUrRAt EARTKtNG 
TRANSFORMER 



PRIME 
MOVER 



EXTRANEOUS 

CONDUCTIVE 

PARTS 




INCOMING ELEC 

BOARD 

].V SUPPLY 



iNOEFENOENT 
rARTH ELECTRODE 

NOTE I — Cable sheath earth of provided/shown 



NOTE 2 — PNE link of provided/shown 






NOTE 3 — If a bus section switch is installed a neutral earthing transformer will be required on each section of 
busbar. 

FIG. 26 Low VOLTAGE STANDBY GENERATORS WITH NEUTRAL EARTHING TRANSFORMERS 



In general, four-pole changeover switching 
between the electricity board supply and the 
standby supply should be used to provide isolation 
of the neutral earths. However, in the case of a 
protective multiple earthing (PME) electricity 
board supply, three- or four-pole switching may 
be used. 

23.2.3.3 Standby generators (capable of parallel 
operation with I he incoming mains supply) — The con- 
ditions for which parallel operation of multiple 
generating set installations with the mains supply 
may be permitted by the electricity board are the 
same as apply for single generators (i^ee 23.2.2.3). 

The possible alternative energy source earth- 
ing arrangements are as listed in 23.2.3.2. 

23.2.4 Single and Multiple Generator Earthing 
(Synchronous Machines) — The parallel operation 
of synchronous machines is generally permitted; 
such machines are normally provided where the 
prime mover is driven by wind, water or bio- 
chemical plant, but may be provided with any 
prime mover. Any neutral point of such machine 
windings should be earthed, but the machine 
framework and any other extraneous metalwork 
should be connected to the electricity board earth 
terminal, if provided. 



23,2,5 Small Portable Low Voltage Generators upto 
5 kVA in Rating — Where portable generators are 
used to provide a supply and earthing is consi- 
dered as a means of protection against electric 
shock, they are required to be connected as 
follows: 

a) Single-phase machines should have either 
a centre tap on the winding connected to 
earth or, if not compatible with the system, 
one end connected to earth and designated 
the neutral. The Centre tap method 
reduces the effective line-earth voltage and 
is particularly used where the generator is 
to feed 1 10 V portable tools; and 

b) Three-phase machines should have their 
windings connected in star, with the star 
connection made available and connected 
to earth. 

In all cases, the exposed metalwork of the 
generator should be adequately connected to the 
earth terminal, preferably with a bolted connec- 
tion. 

The earth electrode should have a minimum 
cross-section area if it is not protected against 
corrosion of 25 mnr for copper and 50 mm" for 
steel. Whilst there is no minimum value of earth 



59 



IS : 3043 " 1987 



electrode resistance, it siiould be as low as possi- 
ble. The upper limit should not exceed the value 
required for the protective devices to operate and 
disconnect the load in a time not exceeding the 
safe value. 

NOTE — The selection of devices for the automatic 
disconnection of supply is covered in Section 3. 

For portable generators, residual current 
devices having an operating time of 40 ms or less 
at a residual current of 250 mA are recommended 
to a means of providing additional protection 
against the effect of electric shock. However, it 
is important to test such devices regularly, parti- 
cularly when the greater is used in a hostile 
environment. The method of connecting a rod 
used on the output of a portable generator is 
shown in Fig. 28. 

23.2.6 Mobile Generators — Where a supply is 
taken from a mobile generator, the following 
recommendations, additional to those given 
in 23.2.5 shall apply: 

a) The generator neutral should be connected 
to the vehicle chassis; 

b) The earth terminal at each outlet on the 



generator vehicle should be connected 
separately to the alternator neutral where 
the latter is bonded to the vehicle chassis; 
and 

c) Where an electricity board protective earth 
terminal or exposed structural metalwork 
is present, it should be connected to the 
earthing conductor on the mobile genera- 
tor. 

23.3 High Voltage Generators 

23.3.1 Earth Electrodes and Earthing Resistors — 
Where an earth electrode resistance is 1 O or 
less, a common earth may be used for the high 
voltage generator and for the low voltage system 
derived through high voltage/low voltage trans- 
formation. 

NOTE — For horther information 5ft? 20.1 (c). 

Where a resistor is used for earthing the star- 
point of a high voltage generator, it is normally 
designed to limit the earth fault current to the 
same order of magnitude as the machine's full 
load current. In general, however earthing via 
resistors is not necessary for single generators of 
I MW or less in rating. 



n y B H 



INCOMING 




EXTRANEOUS 

CONOUCTIVi 

PARTS 



jtf). — 

mWHT 6ENIRAT0R 




'uWr 



INDEFENOENT 
EARTH ElECf 
ROOE 



NOTE 1 — Cable sheath earth of provided'shown ■ 



NOTE 2 -— PNE link of provided/shown 



!#Ei 



point only 



NOTE 3 |fL} Mechanical interlock to ensure that energy source neutral it always earthed but at one 



FIG. 27 Low VOLTAGE STANDBY GENERATORS WITH STAR POINT SWITCHING 

60 



IS : 3043 - 1987 



PRIMi 
MOVER 



OENERATOR 



RC D 





»a--r^^^ 



go- 



-T"*^ 



L,V. PLANT- PORTAatS 
OR MOBILE UNIT 



-LOAD 



FIG. 28 



INDEPENOeNT 
EARTH ELECTRODE 

METHOD OF CONNFXTING A RESIDUAL CURRENT DEVICE (r. 
ON THE OUTPUT OF A PORTABLE OR MOBILE GENERATOR 



d.) 



23.3.2 Single High Voltage Generator Earthing 
(SytTchromus Machines with Star Connected Alterna- 
te rs). 

23.3.2.1 Generator operating in isolation {from 
mains or other suppliers — The star-point of the 
generator should be connected (via a resistor, if 
necessary) and through a bolted link for test 
purposes to an earthing conductor and the inde- 



pendent earth electrode. 

23.3,2.2 Standby generator (without paralleling 
facility) — In addition to the earthing require- 
ments described for a set operating in isolation 
from other supplies, the presence of an incoming 
electricity board supply makes necessary the inter- 
locking of the standby supply circuit breakers to 
prevent inadvertent connection (see Fig. 29). 



I TT T 

r T~T"lNTER-( 






LOCK ^A 



MV sfANoey 

GENERATOR 





INCOMING LOAD 

MAINS 
SUPPLY 



EARTHINS RESISTORS 
IIF REQUIRED) 



FJG. 29 SINGLE HrOH VOLTAGE STANDBY GENERATING SET NOT SUITABLE FOR PARALLEL OPERATION 



61 



IS : 3043 - 1987 



23,3.2.3 Standby generator {capable of parallel 
operation with an incoming supply) — The operation 
of a private generator (or generators) in parallel 
with an electricity board high vohage system is 
subject to the parallel and technical agreement of 
the electricity board. 

In most cases where parallel operation with 
an incoming electricity board is required, an 
earthing contactor is necessary between the 
generator star point and the bolted test link {see 
Fig. 30). The contactor should be interlocked 
with the incoming supply circuit breaker so that 
it is open during periods of parallel operation but 
closes at all times. In the event of the electricity 
supply being lost during a period of parallel 
operation, the earthing contactor should be 
arranged to close automatically. The form of 
generator earthing (direct or resistance) is 
dependent upon the system parameters and the 
machine manufacturer's recommendations. 

23,3.3 Multiple High Voltage Generator Earthing 

23.3.3.1 Generators operating in isolation from 
other supplies — When it is required to operate 
two or more generators in parallel and the method 



of energy source earthing is direct or resistance 
earthing, then earthing contactors should be 
installed between each generator star-point and 
the earthing conductor each electrode (as descri- 
bed in 23.2.3.1). The contactors need to be 
interlocked so that only one can be closed to 
maintain a single energy source earth. 

If a neutral earthing transformer is to be used 
for energy source earthings, it should be connec- 
ted as shown in Fig. 3 1 except that in the case 
of an isolated generating system, the earthing 
contactors is not required. 

23.3.3.2 Standby generators {without mains 
parallel facility) — When the generating sets are 
not to be operated in parallel with the mains 
supply, and have direct or resistance earthing, 
the standby generator circuit-breakers and mains 
circuit-breaker need to be interlocked. 

If a neutral earthing transformer is used the 
requirements are the same as described for a 
single standby generator in 23,3,2.2; as shown in 
Fig. 3 1 , but without the earthing contactor. 




EARTHING COMUCTOR INTERLOCKED WITH 
MAINS SUf»F(.Y CIRCUIT BREAKER 
CAN 8E CLOSED AT 



^CARTHIMO RESISTOH 
IF RiaUtRED 



FIG. 30 SINGLE HIGH VOLTAGE STANDBY GENERATING SET SUITABLE FOR 
PARALLEL OPERATION WITH INCOMING MAINS SUPPLY 



62 



15:3043-1987 



23.3.3.3 Standby generators {capable of parallel 
operation with an incoming mains supply) — When the 
generating sets have direct or resistance earthing 
and are used as standby to the mains, earthing 
contactors are needed if parallel running is a 



requirement. These should be interlocked with 
the incoming mains supply circuit-breaker so that 
they are open during parallel operation of the set 
with the mains, but one is closed at all other 
times {see Fig. 32), 



NOH -AUTOMATIC 
OH SWITCH 



MUTHAt EARTHma 
T9«AI6$FOflM£ll 




5 




LOAD 



FIG. 31 



t iNCOMBNG 
J. MAINS SUPPLY 

V6A«rMIM<P CONTACTOR 
« INTERLOCKED SO THAT THE 

COHTACTOII CANNOT BE CLOSED 
-J DURING PARALLEL OPiRATlOlft 

WITH THE IHCOMtNG MAINS 

SUPPLY 



MULTIPLE HIGH VOLTAGE STANDBY GENERATING SETS WITH NEUTRAL EARTHING 
TRANSFORMER SUITABLE FOR PARALLEL OPERATION WITH EACH OTHER 
AND WITH THE INCOMING MAINS SUPPLY 




EARTMINO m i /|A_®L 

CONTACTOPSS^ •%19" V" "^ 



EARTNUea 
RESISTORS 
Itr REWeilE 




a incoming 
^ Aains supply 



LOAD 



'EARTHIna CONTACTORS INTSRLOCICEO SO THAT 

loWLV ONE CAN @E CLOSED AT ANY TIME AND 

I NONE CAN ®e CLOSED DURINO PARALLEL 

OPERATION WITH THE INC0M3N& MAINS 

SUPPLY 



e 



FIG. 32 MULTIPLE HIGH VOLTAGE STANDBY GENERATING SETS SUITABLE FOR PARALLEL 
OPERATION WITH EACH OTHER AND WITH THE INCOMING MAINS SUPPLY 

63 



IS : 3043 - 1987 



SECTION 7 MEDICAL ESTABLISHMENTS 



24. PROTECTIVE MEASURES THROUGH 
EARTHING IN MEDICAL ESTABLISH- 
MENTS 

24.0 General — In the context of this Section 
"installation", means any combination of inter-' 
connected electrical equipment within a given 
space or location intended to supply power to 
electrical equipment used in medical practice. 

24.0.1 For the puiposes of this Section, 
reference may also be made to SP : 30 (Part 3/ 
Sec4)-1985^ 

24.0.2 As such, some parts of the installation 
may be present in the patient's environment, 
where potential differences, that could lead to 
excessive cun'ents through the patient, must be 
avoided. For this purpose a combination or earth- 
ing of equipment and potential equalization in the 
installation seems to provide the best solution. A 
disadvantage of such a system is that in the case 
of an insulation fauh in circuits directly connected 
to supply mains, the fault current may cause a 
considerable voltage drop over the protective 
earth conductor of the relevant circuit. Since a 
reduction of such a voltage drop by the applica- 
tion of increased cross-sectional areas of protective 
conductors is usually impractical, available solu- 
tions are the reduction of the duration of fault 
currents to earth by special devices or the applica- 
tion of a power supply which is isolated from 
earth. 

24.0.3 Generally a power supply system includ- 
ing a separated protective condiictor is required. 
(TN-S System) in medical estabhshment 
{see 6 A. 1), 

In addition the following provisions may be 
required, depending upon the nature of the 
examinations or treatments performed: 

a) Additional requirements concerning pro- 
tective conductors and protective devices 
to restrict continuous voltage differences. 

b) Restriction of voltage differences by supple- 
mentary equipotential bonding. During 
the appHcation of equipment with direct 
contact to the patient, at least a potential 
equalized zone around thQ patient shall be 
provided with a patient centre bonding bar 
to which the protective and lunctional earth 
conductors of the equipment are connec- 
ted. All accessible extraneous conductive 
parts in the zone shall be connected to this 
potential equalization bar. 

c) Restriction of the potential equalization 
zone around one patient, meaning practi- 
cally around one operation table or around 
one bed in an intensive care room. 



d) If more than one patient is present in an 
area, connection of the various potential 
equalization centres to a central potential 
equalization busher, which should prefer- 
ably be connected to the protective earth 
system of the power supply tor the given 
area. In its completed form, the equipoten- 
tial bonding network may consist partly of 
fixed and permanently installed bonding 
and partly of a number of separate bond- 
ings which are made when the equipment 
is set up near the patient. The necessary 
terminals for these bonding connection 
should be present on equipment and in the 
installation. 

e) Restriction of the duration of transient 
voltage difference by the application of 
residual current operated protective devices 
(earth leakage circuit-breakers). 

f) Continuity of power supply to certain 
equipment in the case of a first insulation 
fault to earth and restriction of transient 
voltage differences by application of isolat- 
ing transformers. 

g) Monitoring of first insulation fault to earth 
in an IT Systems {see 6.1.1) (the secon- 
dary side of an isolating transformer) with 
sufficiently high impedance to earth. 

NOTE — Additional safety measures are required 
besides earthing described in this Section. These 
cover fire safety, safety supply systems and inter- 
ference suppression. Reference may be made to 
NEC (Part 3, Section 4)*. 

24.1 Safety Provisions 

24.1.1 Safety measures from the point of view 
of earthing are divided into a number of provi- 
sions as given in Table 10, 

24.1.2 Provision Po shall be applicable to all 
buildings containing medically used rooms. Provi- 
sion PI shall be appHcable for all medically used 
rooms. 

Other requirements of this Section, need not 
be complied with, if 

a) a room is not intended for the use of medi- 
cal electrical equipment, or 

b) patients do not come intentionally in con-- 
tact with medical electrical equipment 
during diagnosis or treatment, or 

c) only medical electrical equipment is used 
which is internally powered or of protection 
Class 11. 

The rooms mentioned under (a), (b) and (c) 
may be, for example, massage rooms, general 
wards, doctor's examining room (office, consulting 
room), where medical electrical equipment is not 
used. 



^National Electrical Code. 



*National Electrical Code. 



64 



IS : 3043 - 1987 



PROVISIONS 

(0 
PO 



PI 



P2 



P3 



P4 



P5 



TABLE 10 SAFETY PROVISIONS 

(aame 24.1.1) 



PRINCIPAL REQUIREMENTS 

(■2) 
Duration of touch voltage restricted to a safe 
limit 

As PO but additionally : Touch voltages in 
patient environment restricted to a safe 
limit 

As PI but additionally ; Resistance between 
extraneous conductive parts and the protec- 
tive conductor busbar of the room not 
exceeding 0-\ Q 

As PI or P2 but additionally : Potential diffe- 
rence between exposed conductive parts 
and the protective conductor busbar not 
exceeding 10 mV in normal condition (see 
Note) 

As PI or P2. Additional protection against 
electric shock by limitation of disconnect- 
ing time 

Continuity of the mains supply maintained 
in case ofa first insulation fault to earth 
and currents to earth restricted 



INSTALLATION MEASURES 

(3) 
TN-S, TT or IT system (see 6.1.1) 

Additional to PO Supply system with addi- 
tional requirements for protective earth- 
ing, etc 

Additional to PI : Supplementary equipo^ 
tential bonding 



As PI or P2: Measurement necessary, 
corrective action possibly necessary 



Additional to PI or P2 : Residual current 
operated protective device 

Additional to PI, P2 or P3 ; Isolated 
supply system with isolation monitoring 



NOTE — Normal condition means 'without any fault' in the installation. 



24.1.3 Guidance on the application 
provisions are given in Table 11. 



of the 



24.1.4 A typical example of an installation in 
a hospital is given in Appendix C of NEC (Part 3, 
Section 4)* . 

25. SUPPLY CHARACTERISTICS AND 
PARAMETERS 

25.0 Exchange of Information 

25.0.1 Proper coordination shall be ensured be- 
tween the architect, building contractor and the 
electrical engineer or the various aspects of insta- 
llation design. The necessary special features of 
installations shall be ascertained before hand with 
reference to Table 1 1. 

25.1 Circuit Installation Measures for Safety 
Provisions — (See Table 10, col 3). 

25.1.1 Provision PO General 

25.1.1,1 All buildings in the hospital area 
which contain medically used rooms shall have a 
TN-S, TT power system. The conventional touch 
voltage limit (LL) is fixed at 50 V ac. 

NOTE — The use of TN-C-S system (in which the 
PEN-conductor may carry current in normal condition) 
can cause safety hazards for the patients and interfere 
with the function of medical electrical equipment, 
data processing equipment, signal transmission lines, 
etc. 

25.1.2 Provision PI : Medical TN-S System 



'"National Electrical Code. 



25.1.2.1 The conventional touch voltage 
limit (LL) is fixed at 25 V ac. 

25.1.2.2 Protective conductors inside a 
medically used room shall be insulated; their 
insulation shall be coloured green-yellow. 

25.1.2.3 Exposed conductive parts of equip- 
ment being part of the electrical installation used 
in the same room shall be connected to a common 
protective conductor. 

25.1.2.4 A main equipotential bonding with 
a main earthing bar shall be provided near the 
main service entrance. Connections shall be made 
to the following parts by bonding conductors: 

a) lightening conductor; 

b) earthing systems of the electric power 
distribution system; 

c) the central heating system; 

d) the conductive water supply line; 

e) the conductive parts of the waste water line; 

f) the conductive parts of the gas supply; and 

g) the structural metal frame- work of the 
building, if applicable. 

Main equipotential bonding conductors shall 
have cross-sectional areas of not less than half the 
cross-sectional area of the largest protective con- 
ductor of the installation, subject to a minimum 
of 6 mm". The cross-sectional area, need not, 
however, exceed 25 mm" if the bonding conduc- 
tor is of copper or a cross-sectional area affording 
equivalent current-carrying capacity in other 
metals. 



65 



IS : 3043 - 1987 



TABLE 11 EXAMPLES OF APPLICATION OF 
SAFETY PROVISIONS 

(Clause 24.1 3) 



MEDICALLY USED ROOM 



1. Massage room 

2. Operating wash room 

3. Ward, General 

4. Delivery room 

5. ECG, EEG, EMG room 

6. Endoscopic room 

7. Examination or treat- 

ment room 

8. Labour room 

9. Operating sterilization 

room 

10. Orology room (not 

being an operating 
theatre) 

11. Radiological diagnostic 

and tnerapy room, 
other than mentioned 
under 20 and 24 



POTECTIVE MEASURES 



PO/Pl 


P2 


M 





M 


-V 


M 





M 


.V 


K4 


x- 


M 


X 


M 





M 


^ 


M 






P3 



P4 



P5 



M 



M 



12. 


Hydrotherapy room 


M 




13. 


Physiotherapy room 


M 




14. 


Anaesthetic room 


M 


X 


15. 


Operating theatre 


M 


x 


16. 


Operating preparation 
room 


M 


X 


17. 


Operating plaster room 


M 




18. 


Operating recovery 
room 


M 


X 


19. 


Out-patient operating 
room 


M 




20. 


Heart catheterization 
room 


M 


■^ 


21. 


Intensive care room 


M 





22. 


Intensive examination 
room 


M 





23. 


Intensive monitoring 
room 


M 





24. 


Angiographic examina- 
tion room 


M 





25. 


Hemodialysis room 


M 





26. 


Central monitoring 


M 






room {see Note) 

NOTE — Only if such a room is part of a medical 
room group and, therefore, intstalled in the same way as 
an intensive monitoring room. Central monitoring room 
having no conductive connection to the mexially used 
room (for example, by use of isolating coupling devices 
for signal transmission) may be installed as non-medi- 
cally used from (Provision POonly). 



M 



mandatory measure; 
recommended measure; 

as .T, but only for insulation monitoring 
device; and 



additional 
desirable. 



measure may be considered 



25.1.2.5 Each medically used room or room 
group shall have its own protective conductor bus 
bar, which should have adequate mechanical and 
electrical properties and resistance against corro- 
sion. 

This busbar may be located in the relevant 
power distribution box. The leads connected to 
terminals of such a protective conductor bar shall 
be identified and shall be similarly designated on 
drawings of the installation system. 

25.1.2.6 The impedance (Z) between the 
protective conductor bar and each connected 
protective conductor contact in wall sockets or 
terminals should not exceed 2 H, if the rated 
current of the overcurrent-protective device is 
16 A or less. In case of a rated current exceeding 
16 A, the impedance should be calculated using 
the formula: 



in all cases Z shall not exceed 0-2 CI. 

(ly = rated current of overcurrent protective 
device in amperes). 

NOTE — The measurement of the protective con- 
ductor impedance should be performed with an ac 
current not less than 1 A and not exceeding 25 A fom 
a soiuxe of current with a no-load voltage not exceeding 
6 V, for a period of at least 5 a. 

25.1.2.7 The cross-sectional area of the pro- 
tective conductor shall be not less than the 
appropriate value shown in Table 7. 

The cross-sectional area of every protective 
conductor which does not form part of the supply 
cable or cable enclosure shall be, in any case, not 
less than: 

a) 2-5 mm", if mechanical protection is provi- 

ded; and 

b) 4 mm^, if mechanical 
provided. 

25.1.2.8 It may be necessary to run the 
protective conductor separate from the phase 
conductors, in order to avoid measuring problems 
when recording bioelectric potentials. 

25.1.3 Provision P3 ; Supplement cuy Equipotential 
Bonding 

25.1.3.1 In order to minimize the touch 
voltage, all extraneous conductive parts shall be 
connected to the system of protective conductors. 

An equipotential conductor bar shall be provi- 
ded. It should be located near the protective 
conductor bar (.^et? ato 25.1.2.5). A combined 
protective conductor and equipotential bonding 
bar may be used, if all conductors are clearly 
marked according to 25.1.2.5 and 25.1.3.3(e). 

25.1.3.2 Connections shall be provided from 
the equipotential bonding bar to extraneous con- 
ductive parts such as pipes for fresh water, heat- 
ing, gases, vacuum and other parts with a 



protection is not 



66 



IS : 3043 - 1987 



conductive surface area larger than C02 m" or a 
linear dimension exceeding 20 cm or smaller part 
that may be grasped by hand. 

Additionally, the following requirements apply: 

a) Such connections need not be made to: 

1) Extraneous conductive parts inside of 
walls (for example, structural metal 
work of buildings) having no direct 
connection to any accessible conductive 
part inside the room, and 

2) Conductive parts in a non-conductive 
enclosure; 

b) In locations where the position of the 
patient can be predetermined this provision 
may be restricted to extraneous conductive 
parts within the patient environment (see 
Apppendix B of NEC (Part 3, Section 4); 
and 

c) In operating theatres, intensive care rooms, 
heart catheterization rooms and rooms 
intended for the recording of bioelectrical 
action potentials all parts should be connec- 
ted to the equipotential bonding bar via 
direct and separate conductors. 

25.1.3,3 The following requirements shall be 
fulfilled: 

a) The impedance between extraneous conduc- 

tive parts and the equipotential bonding 
bar shall not exceed 1 Q. 

NOTE — The measurement of this impedance 
should be performed with a current not less than 
10 A and not exceeding 25 A during not less than 
5 s horn a current source with a no-load potential 
not exceeding 6 V ac. 

b) All equipotential bonding conductors shall 
be insulated, the insulation being coloured 
green-yellow. 

NOTE — Insulation ofthe equipotential bond- 
ing conductors is necessary, to avoid loops by 
contact and to avoid picking up of stray currents. 

c) Equipotential conductors between perma- 
nently installed extraneous conductive parts 
and the equipotential bonding bar shall 
have a cross-sectional area of not less than 
4 mm~ copper or copper equivalent, 

d) The equipotential bonding bar, if any, 
should have adequate mechanical and 
electrical properties, and resistance against 
corrossion, 

e) The conductors connected to the equipo- 
tential bonding bar shall be marked and 
shall be similarly designated on drawings 
ofthe installation system. 

f) A separate protective conductor bar and an 
equipotential bonding bar in a medically 
used room or in a room group shall be 
interconnected with a conductor having a 
cross-sectional area of not less than 16 mnr 
copper or copper equivalent (see 25.L3.1). 



g) An adequate number (under consideration) 
of equipotential bonding terminals other 
than those for protective conductor contact 
or pins of socket outlets should be provided 
in each room for the connection of an 
additional protective conductor of equip- 
ment or for reasons of functional earthing 
of equipment, 

25.1.4 Provision P3 : Restriction of Touch Voltage 
in Rooms Equipped for Direct Cardiac Application 

25.1.4,1 The continuous current through a 
resistance of 1 000 connected between the equi- 
potential bonding bar and any exposed conductive 
part as well as any extraneous conductive part in 
the patient environment shall not exceed 10 MA 
in normal condition for frequencies from dc to 
1 kHz. 

For a description of patient environment, see 
Appendix B of NEC (Part 3, Section 4). Where 
the measuring device has an impedance and a 
frequency characteristics, the current may also be 
indicated as a continuous voltage with a limit of 
10 mV between the parts mentioned above. 

a) During the test, it is assumed that fixed and 
permanently installed medical electrical 
equipment is operating. 

b) 'Normal conditions' means without any 
fault in the installation and in the medical 
electrical equipment. 

NOTE — To comply with this requirement, it 
may be necessary to apply one or more ofthe 
following methods: 
Extraneous conductive parts may be: 

a) connected to the equipotential bonding bar by 
a conductor of a large cross-sectional area in 
order to reduce the voltage drop across such a 
conductor, 

b) insulated so that it is not possible to touch them 
unintentionally, and 

c) provided with isolating joints at those places 
where they enter and leave the room. 

Exposed conductive parts of permanently 
installed equipment may be isolated from the 
conductive building construction. 

25.1 .5 Provision P4: Application of Residual-Current 
Protective Devices 

25.1.5.1 The use of a residual-current pro- 
tective device is not recognized as a sole means of 
protection and does not obviate the need to apply 
the provisions PI and P2. 

25.1.5.2 Each room or each room group 
shall be provided with at least one residual-current 
protective device. 

25.1.5.3 A residual-current protective device 
shall have a standard rated operating residual^ 
current/AiV < 30 mA. 

25.1.5.4 A medical isolating transformer and 
the circuits supplied from it shall not be protected 
by a residual current protective device. 



67 



IS : 3043 - 1987 



25.1.5.5 Electrical equipment, for example, 
general lighting luminaries, installed more than 
2»5m above floor level, need not be protected by 
a residual- current protective device. 

25.1.5.6 Fixed and permanently installed 
electromedical equipment with a power consump- 
tion requiring an overcurrent protective device of 
more than 63 A rated value may be connected to 
the supply mains by use of a residual-current 
protective device with I A N< 300 mA. 

25.1.6 Provision P5: Medical IT System 

25.1.6.0 The use of a medical IT-System for 
the supply of medically used room for example, 
operating theatres, may be desirable for different 
reasons: 

a) A medical IT-System increases the reliabi- 
lity of power supply in areas where an 
interruption of power supply may cause a 
hazard to patient or user; 

b) A medical IT-System reduces an earth fault 
current to a low value and thus also 
reduces the touch voltage across a protec- 
tive conductor through which this earth 
fault current may tlow; 

c) A medical IT-System reduces leakage 
currents of equipment to a low value, 
where the medical IT-System is approxi- 
mately symmetrical to earth. 

It is necessary to keep the impedance 
to earth ofthe medical IT-System as high 
as possible. This may be achieved by: 

a) restriction ofthe physical dimensions 
ofthe medical isolating transformer, 

b) restriction of the system supplied by 
this transformer, 

c) restriction of the number of medical 
electrical equipment connected to such 
a system, and 

d) high internal impedance to earth of 
the insulation monitoring device con- 
nected to such a circuit. 

If the primary reason for the use of a medical 
IT-System is the reliability ofthe power supply, 
it is not possible to define, for such system, a 
hazard current and an insulation resistance moni- 
toring device should be used. 

If, on the other hand, the restriction of leakage 
current of equipment is the main reason for the 
use of the medical IT-System, an insulation 
impedance monitoring device should be used. 

25.1.6.1 For each room or each room group 
at least one fixed and permanently installed 
medical isolating transformer shall be provided. 

25.1.6.2 A medical isolating transformer 
shall be protected against short-circuit and over- 
load. 



In case of a short-circuit or a double earth 
fault in parts of opposite polarity of the medical 
IT-System, the defective system shall be discon- 
nected by the relevant overcurrent protective 
device. 

If more than one item of equipment can be 
connected to the same secondary winding of the 
transformer, at least two separately protected 
circuits should be provided for reasons of conti- 
nuity of supply. 

25.1.6.3 Overcurrent protective devices shall 
be easily accessible and shall be marked to indicate 
the protective circuit. 

25.1.6.4 An insulation monitoring device 
shall be provided to indicate a fault of the insula- 
tion to earth of a live part of the medical 
IT-System. 

25.1.6.5 Fixed and permanently installed 
equipment with a rated power input of more than 
5 kVA and all X-ray equipment (even with a 
rated power input of less than 6 kVA) shall be 
protected by Provision P4. Electrical equipment, 
for example, general lighting, more than 2*5 m 
above floor level, may be connected directly to 
the supply mains. 

25.1.6.6 General requirements for insulation 
monitoring devices — A separate insulation resistance 
or impedance monitoring device shall be provided 
for each secondary system. It shall comply with 
the requirements of (a) to (d) below: 

a) It shall not be possible to render such a 
device inoperative by a switch. It shall 
indicate visibly and audibly if the resistance 
or impedance of the insulation falls below 
the value given in 25.1.6.7 and 23.1.6.8. 

b) A test button shall be provided to enable 
checking the response of the monitor to a 
fault condition as described in 25.1.6.4. 

c) The visible indication mentioned in (a) of 
the insulation monitoring device shall be 
visible in the monitored room or room 
group. 

d) The insulation monitoring device should be 
connected symmetrically to the secondary 
circuit ofthe transformer. 

25.1.6.7 Insulation resistance monitoring device — 
The ac-resistance of an insulation resistance 
monitoring device shall be at least 100 kQ. The 
measuring voltage of the monitoring device shall 
not exceed 25 V, and the measuring current 
(in case of a short-circuit of an external 
conductor to earth) shall not exceed 1 mA. The 
alarm shall operate if the resistance between the 
monitored isolated circuit and earth is 50 kO or 
less, setting to a higher value is recommended. 

25.1.6.8 Insulation impedance monitoring device — 
An insulation-impedance monitoring device shall 



68 



18:3043-1987 



give reading calibrated in total hazard current 
with the value of 2 mA near the centre of the 
metre scale. 

The device shall not fail to alarm for total 
hazard currents in excess of 2 mA. In no case, 
however, shall the alarm be activated until the 
fault hazard current exceeds 0-7 mA. 



NOTE — The value of 2 mA or 0-7 mA are based 
on practical experience with 110 to 120 V power supp- 
lies. For a 220-240 V power supply, it may be necessary 
to increase these values to 4 and 1 -4 mA because of the 
higher leakage current of equipment. 

During the checking of the response of the 

monitor to a I'ault condition the impedance 

between the medical IT-System and earth shall 
not decrease. 



SECTION 8 STATIC AND LIGHTNING PROTECTION EARTHING 



NOTE — For the time bein^, the general principles 
ofstatic and lightning protection earthing, together 
with the relevant rules for such purposes as contained in 
13:7689-1974 'Guide for control of undesirable static 
electricity' and IS : 2309-1969 'Code of practice for the 
protection of buildings and allied structures against ligh- 
tning {first revisiony are considered as valid in this section. 



A simultaneous review/revision 
is in progress. 



of these standards 



For completeness of the earthing code, it is propos"^ 
edto include relevant earthing and bonding details for 
control ofstatic electricity and lightning protection in 
Section 8 in due course. 



SECTION 9 MISCELLANEOUS INSTALLATIONS AND CONSIDERATIONS 



28. EARTHING IN POTENTIALLY HAZAR- 
DOUS AREAS 

28.1 Earthing and Bonding 

28. LI Earthing should be in accordance with 
the relevant sections of this code. The connection 
between metal part to be grounded and the groun- 
ding conductor shall be made secure mechanically 
and electrically by using adequate metallic fitting. 
The grounding conductors shall be sufTiciently 
strong and thick, and the portions of conductor 
which are likely to be corroded or damaged shall 
be well protected. Grounding conductors which 
shall not reach a hazardous high temperature due 
to the anticipated maximum earth fault current 
flowing shall be used. 

28, L2 Protection against lightning shall be 
provided in accordance with Section 8. Specific 
guidelines for installations in hazardous locations 
are given in IS : 2309-1969* Inter-connection 
system with other buried metal services and/or 
earth terminations for equipment grounding for 
the purpose of equalizing the potential distribution 
in the ground should preferably be made below 
ground. 

28.1.3 Portable and transportable apparatus 
shall be grounded with one of the cores of flexible 
cable for power supply. The earth continuity con- 
ductor and the metallic screen, wherever provided 
for the flexible cable, should be bonded to the 
appropriate metalwork of the apparatus and to 
earthing pin of the plug. 

28.1.4 Efficient bonding should be installed 
where protection against stray currents or electro- 
static charges is necessary. 

28.1.5 Earthing and Bonding of Pipelines and Pipe 
Racks — Unless adequately connected to earth 



*Code of practice for the protection of buidings and 
allied structures against lighting {first revision). 



elsewhere, all utility and process pipelines should 
be bonded to a common conductor by means of 
earth bars or pipe clamps and connected to the 
earthing system at a point where the pipelines 
enter or leave the hazardous area except where 
conflicting with the requirements of cathodic pro- 
tection. In addition, it is recommended that steel 
pipe racks in the process units and off-site areas 
should be grounded at every 25 m. 

28.2 Permissible Type of Earthing System 

28.2.1 Guidance on permissible power systems 
is given below: 

a) if a power system with an earthed neutral 
is used, the type TN-S system with separate 
neutral (Af) and protective conductor (PE) 
throughout the system is preferred. 

The neutral and the protective conductor 
shall not be connected together or com- 
bined in a single conductor in a hazardous 
area. 

A power system of type Indian TN-C (ha- 
ving combined neutral and protective 
functions in a single conductor throughout 
the system) is not allowed in hazardous 
area. 

b) If a type IT power system (separate 
earths for power system and exposed con- 
ductive parts) is used in Zone 1, it shall be 
protected with a residual current device 
even if it is a safety extra-low voltage cir- 
cuit (below 50 V). 

The type TT power system is not permitted 
in Zone 0. 

c) For an IT power system (neutral isolated 
from earth or earthed through impedance), 
an insulation monitoring device should be 



69 



IS : 3043 - 1987 



used to indicate the first earth fault. How- 
ever, equipment in Zone shall be discon- 
nected instantaneously in case of the first 
earth fault, either by the monitoring device 
or by a residual current operated device. 

d) For power systems at all voltage levels in- 
stalled in Zone 0, due attention should be 
paid to the limitation of earth fault currents 
in magnitude and duration. Instantaneous 
earth fault protection shall be installed. 

It may also be necessary to provide instantane- 
ous earth fault protection devices for certain 
applications in Zone I. 

28.2.2 Potential Equalization — To avoid dan- 
gerous sparking between metallic parts of struc- 
tures, potential equalization is always required for 
installations in Zone and Zone I areas and may 
be necessary for installations in Zone 2 areas. 
Therefore, all exposed and extraneous conductive 
parts shall be connected to the main or supple- 
mentary equipotential bonding system. 

The bonding system may include normal pro- 
tective conductors, conduits, metal cable sheaths, 
steel wire armouring and metallic parts of struc- 
tures but shall not include neutral conductors. 
The conductance between metallic parts of struc- 
tures shall correspond to a cross-section of at least 
10 mm" of copper. 

Enclosures are not to be separately connected 
to the equipotential bonding system if they are 
secured to and are in metallic contact with struc- 
tural parts or piping which are connected to the 
equipotential bonding system. 

For additional information, see relevant section 
of this code. 

However, there are certain pieces of equipment, 
for example, some intrinsically safe apparatus, 
which are not intended to be connected to the 
equipotential bonding system. 



29. TELECOMVIUNICATION 
AND EQUIPMENT 



CIRCUITS 



29.1 General — In addition to protective ear^ 
thing which may be required in accordance with 
this code, telecommunication systems may require 
functional earths for any or all of the following 
purposes: 

a) to complete the circuits of telegraph or 
telephone systems employing on-earth path 
for signalling purposes; 

b) to earth the power supply circuit and sta- 
bilize the potential of the equipment with 
respect to earth; 

c) for lightning-protective apparatus; and 

d) to earth screening conductors to reduce 
electrical interference to the telecommuni- 
cation circuits. 



If equipment requires both a protective earth 
and a functional earth connection, it is preferred 
that the two earths should be separated within the 
equipment so that power system fault currents 
cannot flow in the functional earthing conductors. 
The functional earthing system and conductors 
can then be designed solely in accordance with 
the requirements of the telecommunication system. 
Alternatively, the protective and functional earth 
may be connected together within the equipment 
but in this case the functional earth system and 
conductors should be suitable for the current they 
may carry under power system fault conditions. 

The general recommendations for lightning 
protection apply to earth systems for telecommuni- 
cation lightning protection. 

The telecommunication functional earth should 
be obtained from a point which even under power 
system fault conditions is unlikely to have a 
dangerous potential to remote earth. 

The consumer's earth terminal of a TN system 
is suitable, otherwise a suitable earth electrode 
system, separate from the protective earth, should 
be provided. 

29.2 Telecommunication Circuits Association with High 
Voltage Supply Systems — Telecommunication cir- 
cuits used in any way in connection with or in 
close proximity to high voltage equipment require 
special attention and due consideration should 
be given to the safeguarding of such circuits 
against rise in potential of the supply system earth- 
electrodes. 

When a telecommunication circuit is provided 
in a building, where a high voltage system termi- 
nates and the telecommunication circuit is part of 
or is electrically connected to a system outside 
the 'earth-electrode area', precautions should be 
taken to safeguard personnel and telecommunica- 
tion plant against rise of potential of the earth- 
electrode system. 

The term 'earth-electrode system' includes all 
metalwork, such as power cable sheaths, pipes, 
frameworks of buildings and metal fences, bonded 
to the power system earth electrodes and situated 
within a distance of 100 m outside the fencing 
that surrounds the high voltage compound or 
compounds; it also includes the first three supports 
of any overhead line leaving the station. The 
'earth-electrode area' is any area within 5 m of 
any part of the earth-electrode system. 

The following practice is recommended: 

a) In all cases as great a separation as is 
practicable should be provided between 
the telecommunication cables and the sta- 
tion earth-electrode system. Nevertheless, 
within a station, to prevent the appearance 
of potential differences between normally 
accessible metal parts, all such parts of the 
telecommunication installation should be 



70 



IS : 3043 - 1987 



connected to the station earth-electrode 
system. 

b) At stations where the neutral of the high 
voltage system is earthed, it is generally 
practicable from a knowledge of the im- 
pedance of the earth-electrode system and 
of the maximum earth-fauit current to 
estimate the rise of earth potential that will 
occur upon the incidence of a fault. Where 
the estimate does not exceed safe values no 
precaution additional to that described in 
(a) is necessary. This limit may be extended 
to higher values if all the power lines con- 
tributing to the earth fault current are in 
the 'high-reliability' category. 

If the estimate is above safe limits (see 
20.5.1), the following additional precau- 
tions should be observed. 

c) Where the telecommunication circuit lies 
within the 'earth-electrode area', it should 
be run in insulated cable capable of with- 
standing the application of a test voltage of 
2200 V dc (or ac 50 Hz peak) or (1 500 + 2(7) 
V dc (or ac 50 Hz peak), where U 
is the estimated rise of earth potential, 
whichever is the greater, between conduc- 
tors and earth for 1 min. It is preferred 
that the cables have no metallic sheath, 
armouring or screen but, if any exists, it 
should be isolated either from the rise of 
earth potential or from the rest of the 
telecommunication network by insulation 
capable of withstanding the above test 
voltage. The station terminal equipment 
and wiring should be isolated from the line 
by a barrier designed to withstand the test 
voltage as above. All wiring and apparatus 
connected to the line side of this barrier 
should be insulated from the station earth 
to withstand the same test voltage. 

d) Any earth connection for the telecommuni- 
cation circuit required on the line side of 
isolating barrier should be obtained from a 
point outside the earth electrode area via 
either a pair in the telecommunication 
cable or a cable insulated in accordance 
with (c). 

In practice, (c) and (d) are normally confined 
to stations where the neutral of a 33 kV or higher 
voltage system is earthed since, at other stations, 
line faults do not usually produce dangerous 
conditions. 

30. BUILDING SITES 

30.1 In the often damp and rough environment 
of building sites, precautions to prevent electrical 
hazards have to be robust and regularly inspected 
and this particularly applies to the earthing system. 

Because of the great difficulty of ensuring that 
all incoming metallic services and extraneous 
metalwork are bonded to the neutral of the supply 
system, where the supply is at 4 15 V/240 V, to 
thus satisfy the requirements of thePME approval, 



it is unlikely that the supply authority will offbr 
an earth terminal where the supply system has a 
multiple earthed neutral. If the supply is at a 
voltage higher than 415 V, the developer will 
have to provide the neutral earthing on the low 
voltage system. 

30.2 The main protection against electrical 
hazards on a construction site is the use of a 
reduced low voltage system fbr power tools (110 
V between phases and 55 V to mid-point earth 
or 65 V to star-point earth) and safety extra low 
voltage for supplies to headlamps, etc. 

The earth fault loop impedances on a reduced 
voltage system or on a 240/415 V system serving 
fixed equipment should allow disconnection with- 
in the safe duration. 

30.3 Early discussions with the electricity board 
are essential so that agreement on the type of 
supply, including earthing facilities, can be 
obtained. Where the supply is provided from the 

■ low voltage distribution system, the increasing 
use of protective multiple earthing (PME) will 
usually prevent an earthing terminal being provi- 
ded by the electricity board because of the develo- 
pers inability to comply with the requirements of 
the PME approval during construction work. 

31. MINES AND QUARRIES 

31.1 General — Earthing requirements for 
mines and quarries are based on the broad prin- 
ciple that exposed conductive parts of apparatus 
should be efficiently connected to earth or other- 
wise protected by other equally effective means 
to prevent danger resulting from a rise in poten- 
tial (above earth) on these conductive parts. 

In some mines and certain quarries (quarries 
include open cast coal sites), in addition to shock 
risk, there are also dangers associated with the 
possible presence of flammable gas and explosive 
materials. In these cases, separate local earthing 
may be necessary to avoid incendive sparks caused 
by static electrical discharge. 

31.2 Power System Earthing — At most mines 
and quarries, the incoming supply is provided by 
the supply authority who will instal switchgear 
and metering for their own purpose. It is impor- 
tant to clearly establish in all cases, the point at 
which the supply authorities' responsibilities ter- 
minate and where the consumer's responsibility 
commence. 

If the supply is from a transformer (or 
generator), that is, the property of the supply 
authority, and is on site, a request should be 
made for them to facilitate connection of the 
consumer's earthing system to the neutral or mid- 
voltage point. In some cases, the supply authority 
will allow the use of their earth electrode for 
joint use, in this event the consumer may not 
have to provide and maintain his own earth 
electrode. If the supply is from a transformer 



71 



IS : 3043 - 1987 



that is not the property of the supply authority, 
or if the consumer generates electricity privately, 
then the consumer should provide and maintain 
the earth electrodes that have the neutral or mid- 
voltage points bonded to them. 

If the supply transformer (or generator) is 
distant from the consumer's premises, provision 
of an earth terminal at the premises should be 
requested. Where this is possible, the earth termi- 
nal should be made available by means of an 
additional earth conductor in the supply cable or 
overhead line. 

NOTE — The supply cable sheath and armouring 
may serve the purpose of this earth conductor provi- 
ded that they are bonded to the supply source earth, 
neutral or mid-voltage point and meet the 50 percent 
conductivity requirement. 

If the provision of such an earth terminal is 
impracticable, then it is imperative that the earth 
electrodes at the supply source and consumers' 
premises are maintained such that their resistance 
to the general mass of earth is as low as possible, 
for example, less than 2 Q, and appropriate earth 
fault protection is provided. 

In all cases, the aim should be to maintain 
earth electrode resistance, as low as is practicable, 
taking account of the site conditions, for example, 
soil/rock resistivity. Except, however, for the 
instance quoted above, the achievement of a low 
resistance is not so important as adequate bond- 
ing of all exposed metallic parts back to the 
supply source neutral or mid-voltage point earth 
electrode. 

The mains supply system neutral or mid- 
voltage points should be earthed at one point 
only and in the case of mines, this should be on 
the surface. The connection to earth may either 
be a solid connection or via an impedance to 
limit the prospective earth fault current and in 
the case of impedance earthed systems, suitable 
earth fault provided, that is, capable of detecting 
the restricted flow of fault current. 

No switch or circuit-breaker or fuse should be 
placed in any earthing conductor, although an 
interlocked changeover linking device is allowed 
in certain cases where two or more earth electro- 
des are provided. Such a device would be used 
to allow periodic testing of an electrode resistance 
to the general mass of earth. 

31.3 Apparatus Earthing at Coal and Other 
Mines — Every metallic covering of any cable 
should be earthed. This may be considered as 
forming part of the earthing conductor except in 
the case of flexible trailing cables where specific 
earthing conductors may also be required. 

Earthing conductors installed for that purpose 
should have a conductivity throughout (includ- 
ing j oints) of not less than half that of the conduc- 
tor having the greatest current carrying capacity, 
to which that earth conductor is related and 



should have a cross-sectional area of not less than 
14 mm^, in the case of flexible cable working at 
less than 125 V, the cross-section area need not 
be greater than 6 mm^; also a flexible cable on 
the surface of the mine supplying a load less than 
3 kW need not have an earth conductor larger 
than the power conductors. 

Cables incorporating steel tape armour (unless 
supplementing steel wire), aluminium armour or 
copper sheathed (mineral insulated) cables are 
unsuitable for use below ground. Generally single 
or double, steel wire armoured cables are used. 
The use of paper-insulated lead covered cable is 
also discouraged from use below ground owing to 
the poor mechanical strength of the paper insulat- 
ing material. 

The following are released from the require- 
ments to be earthed, when used solely at the 
surface of the mine: 

a) any lamp holder, that is, efficiently protec- 
ted by a covering which is insulated or 
earthed and made of fire resisting material; 

b) any hand held tool that is double insula- 
ted; 

c) any portable apparatus working at less 
than 50 V dc or 30 V ac; and 

d) any other non-portable apparatus working 
at less than 250 V dc or 125 V ac. 

In the case of electrical circuits used for con- 
trol, interlocking and indicating instruments, the 
regulations allow one pole of the auxiliary trans- 
former secondary winding serving these circuits to 
be connected to earth as an alternative to mid^ 
point earthing. 

Where mobile apparatus containing its own 
source of electricity, for example, mobile genera- 
tor sets and diesel-electric vehicles/cranes, is used 
on the surface, then an exception is required from 
the present regulations if the requirement to earth 
these to the main earth electrode is impracticable. 
However, the bonding together of all exposed 
metallic parts is required. 

New regulations are proposed which, it is 
hoped, will eliminate this anomally by calling for 
all parts of such apparatus to be securely bonded 
together to prevent danger and relex the require- 
ment to connect the structure to the main earth 
system. 

Below ground, where self-contained mobile 
apparatus is used, for example, battery locomo- 
tives, these should be operated as totally insulated 
systems (to avoid sparks between metal parts of 
the apparatus). Warning systems should be 
provided to give an indication of leakage to 
frame. 

At places below ground, where flammable gas 
may occur in quantity to indicate danger 
(usually deemed to be places where 0-25 percent 



72 



IS : 3043 - 1987 



flammable gas could be present in the general 
bodyofair), then limitation of the maximum 
prospective earth fault current is called for on 
power systems working at voltages between 250 
and 1 200 V (the range of voltage normally used 
for coal winding machinery served by flexible 
trailing cables). In these cases, the maximum 
prospective earth fault current should be limited 
(normally by impedance earthing) to 16 A at 
voltages between 250 and 650 V and to 2 A at 
voltages between 650 and 1 200 V. In either case, 
the switchgear contorlling the circuit should be 
able to detect and cut-off the supply of electricity 
with less than one-third of the maximum prospec- 
tive earth fault current flowing. 

NOTE — The ratio between maximum prospective 
earth fault current and protection settings is known as 
the 'tripping ratio'. In practice it has been found that 
in order to take account of voltage depressions occurr^ 
ing when a short circuit coincides with an earth fault 
the tripping ratio should beset to at least 5 : I. Multi-' 
point earthing of a power circuit (sometimes referred 
to as an 'insulated' or 'free neutral system') is allowed 
at any place in a mine, including places where flam- 
mable gas may occur, provided tnat a transformer is 
used which has a means to cut off the supply and 
prevent danger should a breakdown occur between the 
primary and secondary windings. In these systems the 
maximum prospective earth fault current does not 
usually exceed 2 A and switchgear is set to trip at less 
than one-fifth of this value. 

Signalling and telephone circuits may be con- 
nected to earth where safety is enhanced and the 
method of connection is approved by the concer- 
ned authority for that type of apparatus. 

31.4 Apparatus Earthing at Miscellaneous 
Mines and Quarries — Every earthing conduc- 
tor should have an equivalent cross-sectional area 
of not less than 14 mm" except this requirement 
does not apply to an earthing conductor, that 
is: 

a) the metallic covering of a cable, which 
should have conductance not less than half 
that of the largest current carrying capacity 
conductor in that cable; 

b) one of the conductors in a multi-core flexi- 
ble cable used to supply portable appa- 
ratus, in which case the earth conductor 
has to be equal in cross-sectional area to 
that of the largest current carrying conduc- 
tor; and 

c) a part of an overhead line on the surface 
which should have a cross-sectional area of 
not less than 12 mnr. 

Every cable at a miscellaneous mine 
or quarry operating at voltages exceeding 
250 Vdc or 125 V ac, other than flexible 
cables and those not required to be covered 
by insulating material, should be protected 
throughout by a suitable metallic covering 
that has to be earthed. Metallic covering 
is defined in the regulations and it should 
be noted that this does not include any 



metals other than iron or steel, therefore 
cables with armourings or metallic cover 
made of soft metals such as aluminium and 
copper (Mice cable) cannot be used on 
these premises where the voltages exceed 
250 V dc or 125 V ac. 

Where a cable is provided with a lead sheath, 
in addition to the required 'metallic' covering, 
the conductance of the lead sheath may be taken 
as contributing to that of the metallic covering. 
For such installations, plumbed joints have to be 
used where the lead sheath is jointed or termi- 
nated. 

Where flexible cable is used to supply portable 
apparatus at voltages exceeding 250 V dc or 125 
V ac, such cable should be protected by one of 
the following: 

a) A metallic covering (flexible wire armour- 
ing) that encloses all the conductors and 
having a conductance of not less than half 
that of the largest current carrying conduc- 
tor, or where this is impracticable, having 
a conductance not less than that of a 14 
mm" cross-sectional area copper conductor. 

b) A screen of wires to enclose all the conduc- 
tors (collectively screened type cable) 
having a conductance not less than that of 
a 14 mm" cross-sectional area copper con- 
ductor. 

c) A screen of wires arranged to individually 
enclose each conductor (individually 
screened type cable), other than the earth 
conductor. Cables of this construction for 
use in quarries have to be approved by 
HSE. For miscellaneous mines, the screens 
should each have a conductance of not 
less than that of 6 mm' cross-sectional area 
copper conductor. 

Where flexible cables are used with 
portable apparatus at quarries and the size 
of the conductor is such as to make the use 
of one multicore cable impracticable, 
single core cables of such construction and 
bonded in such a manner as HSE may 
approve, may be used. 

32. STREET LIGHTING AND OTHER 

ELECTRICALLY SUPPLIED STREET 

FURNITURE 

NOTFi — Street furniture includes fixed lighting 
cohimns, illuminated traftlc signs, bollards and other 
electrically supplied equipmen't permanently placed in 
the street. 

32.1 In all cases the local supply authority should 
be consulted before design work on new street 
furniture is commenced to ascertain the type of 
system that will supply the new installation. 

32.2 Street furniture may be fed from the circuit 
protected by a TN-S system and in such arrange- 
ments a supply cable with separate phase, neutral 



73 



IS : 3043 - 1987 



and protective conductor is required, that is, an 
SNE cable. The wiring on the load side of the 
protective device in the unit should consist of se- 
parate phase, neutral and circuit protective con- 
ductors. Exposed extraneous conductive parts of 
the item of street furniture being supplied should 
be bonded to the earthing terminal within the 
equipment. The earthing tetminal is itself connec- 
ted to the supply protective conductor. 

32.3 An alternative method of supplying and 
protecting street furniture is by means of a T-C-S 
system. In such cases, a combined neutral and 
earth conductor cable is noraially used, that is, a 
CNE cable. 

32.4 Wiring on the load side of the protective 
device in the units being supphed should use, 
unless a special approval has been obtained, 
separate phase, neutral and circuit protective con- 
ductors. Exposed extraneous conductive parts 
should be bonded to the neutral terminal by a 
conductor with a copper equivalent cross-section 
of 6 iTun or the same as that of the supply neutral 
conductor if this is less. This requirement does not 
apply to small isolated metal parts not likely to 
come into contact with exposed metallic or extra- 
neous metal parts or with earth, for example, 
small metallic doors and door frames in concrete 
or plastics units should not be so connected. 

32.5 In the case of circuits feeding more than one 
item of street furniture, for example, by looping, 
an earth electrode should be installed at the last 
or penultimate unit and this electrode should be 
such as to make the resistance to earth of the 
neutral at any point less than 20 Q before the 
connection of any circuit protective or bonding 
conductors to the neutral terminal. Should the 
provision of one electrode resuh in not meeting 
the 20 Q requirement other earth electrodes equ- 
ally spaced along the circuit have to be installed. 
Alternatively, the earth electrode may be omitted 
if it is possible to connect the neutral at the uhi- 
mate unit to a neutral connected to a different 
supply system. 

There are two further possibilities that may 
arise: 

a) where the supply system is TN-C but where 
the lighting authority wishes to use SNE 
cable in the installation and does not wish 
to use the supply authority's CNE con- 
ductor as a fault path, and 

b) where the supply authority does not pro- 
vide an earth terminal. 

32.6 In both of these cases, the hghting authority 
should provide its own protective earthing elec- 
trode and the system will be the TT-system. Care 
is necessary to ensure that both the initial and 
continuing impedance of the fault path is suffici- 
ently low to ensure the operation of the protective 
device on the occurrence of a fault in the fixtures. 
The neutral earth electrode at the supply trans- 
fonner is an important part of the fault loop but 



its resistance to earth is not under the control of 
the lighting authority. In such circumstances, 
consideration should be given to the use of resi- 
dual current devices to ensure disconnection of 
faulty equipment. 

The use of metallic street light columns or the 
metal carcasses of control units, etc, as protective 
earth electrodes is not recommended. 

33. EARTHING OF CONDCTORS FOR SAFE 
WORKING 

33.1 General — This clause deals only with the 
broad principles of the earthing of conductors for 
safety purposes. It is intended to cover the safety 
earthing of both light and heavy current equip- 
ment and is generally applicable to high voltage 
equipment; however, in some circumstances it 
may, where required, be applied as an additional 
safety feature to low voltage equipment. Where 
applicable, the use of safety earths should be part 
of overall safe system of work, which will include 
isolation, locking off, permits to work or similar 
docuiTients and liaison between parties in control 
of the supplies and in control of the work. To 
ensure that a safe system of work is clearly set out, 
a set of detailed rules and procedures will be 
necessary in each particular case. 

33.2 Safety Earthing — When maintenance or 
repair work, etc, is to be undertaken on or near 
to high voltage apparatus or conductors, precau- 
tions in connection with safety earthing should be 
taken generally as indicated below. All phases or 
conductors of any apparatus or main to be worked 
on should be made dead, isolated and earthed and 
should remain earthed until work is completed. 
Due regard should be taken of changing conditions 
during the progress of work which may necessitate 
revision of earthing arrangements to ensure the 
continuous of safety measures, for example, if a 
connection is made to another source of supply, 
whilst work is in progress, then additional earths 
would be necessary as work proceeds. 

Safety earthing equipment may be available as 
permanent equipment, such as earthing switches, 
as part of permanent equipment such as provision 
for integral earthing of a circuit breaker, or as 
portable earthing equipment such as portable 
earthing leads. All such equipment needs to recei- 
ve regular maintenance and should be inspected 
before use. 

Wherever possible, initial earthing should be 
carried out via a circuit-breaker of other suitable 
fault-rated device. 

Earthing leads should, in every case, be of 
adequate cross-sectional area to carry with safety, 
during the time of operation of the protective 
devices, the maximum short-circuit current that 
may flow under fault conditions. If possible, they 
should either be flexible, braided or stranded bare 
copper conductors or aluminium conductors suita- 
bly protected against corrosion and mechanical 



74 



IS : 3043 - 1987 



damage. In no case, even for the earthing of 
light current equipment (for example, high vol- 
tage testing equipment), should the cross-sectional 
area of the earthing lead be less than 6 mm. 

It has been found in some cases that a 70 mm' 
copper equivalent earthing lead is the largest that 
can be conveniently handled. In such cases, where 
a larger size of lead is necessary to carry with 
safety, the maximum short-circuit current that can 
occur, it may be necessary to use a number of 
leads of 70 mm^ or other suitable size in parallel. 

Before earthing leads are applied, it should be 
verified that the circuit is dead and, where appli- 
cable, a test by means of a suitable type of voltage 
indicator should be applied (the indicator itself 
being tested immediately before and after verifi- 
cation) before applying earth connections. 

Earthing leads should first be efficiently bolted 
or clamped to the permanent earthing system or 
to a substantial electrode of low resistance. Should 
no convenient permanent earth electrode be readily 
available, a substantial copper earth-spike driven 
well into the ground can be utilized to provide a 
quick and convenient temporary earth electrode, 

Whilst such a spike is not generally adequate 
as a primary safety earth, it will give a degree of 
protection against energizing by induction. 

Earthing leads should then be securely bolted 
or clamped to apparatus of conductors to be wor- 
ked on and these connections should be removed 
in all cases before the earthing leads are disconnec- 
ted from the earth electrode or earthing system. 

A suitable insulated earthing pole or device 
should be used to apply earthing leads to appara- 
tus or conductors on which work is to be under- 
taken. 

Earthing leads should be kept as short as 
possible and be placed in such a position that 
they cannot be accidently disconnected or distur- 
bed whilst work is in progress. 

33.3 Precautions Relating to Apparatus 
and Cables — In the case of switchgear, phases 
of the section in which the work is to be done 
should be short-circuited and earthed to the same 
earthing system. Self-contained or portable appara- 
tus is generally available for this purpose. Wher- 
ever possible, automatic tripping features of circuit 
breakers should be rendered inoperative by being 
disconnected ftom the tripping battery before the 
circuit-breaker is closed and the breaker operating 
mechanism should be locked in the closed position. 

With transformers, if there is any possibility of 
any winding becoming inadvertently live, the 
terminals of all windings should be earthed so that 
no danger from shock can occur. When the neu- 
tral points of several transformers are connected 
to a common bar, which is then earthed through 
a resistance of an arc suppression coil, the neutral 



point of any transformer that is to be worked on 
should be disconnected and directly earthed as 
well as the phase terminals. 

When liquid earthing resistors are to be worked 
on, particularly when they are drained for work 
inside, the central electrode should be shorted to 
the tank and not earthed remotely. This is especi- 
ally important where two liquid resistors are 
located side-by-side and one remains in commis- 
sion while the other is opened for maintenance. 

When work is to be carried out on equipment 
that is capable of capacitively storing electrical 
energy, for example, cables and capacitors, such 
equipment has to be discharged to earth prior to 
work cornmencing. As, in some circumstances, 
charge can reappear on such apparatus without re- 
connecting it to a source of supply, it is important 
work that the equipment should remain earthed 
whilst is in progress. The cutting of a cable during 
the course of work may disconnect conductors from 
safety earths and precautions should be taken to 
prevent this happening. 

33.4 Precautions Relating to Overhead 
Lines — After a line has been made dead, isolated, 
discharged and earthed at all points of supply, a 
working earth should be securely attached to each 
phase of the line at the point or points where work 
is to be carried out. 

The provision of a working earth entails a 
connection to a continuous earth wire or to a 
temporary earth electrode, the resistance of which 
need not be low. The application of earths to all 
phase conductors will, in addition to earthing the 
conductors, apply a short-circuit to all phases. 

The connection of the earthing lead to each 
conductor of the overhead line should be made 
using a suitable mechanical clamp placed round 
the conductor by means of an insulated earthing 
pole which can also be utilized to secure the clamp 
tight round the line conductor. When it is requi- 
red to remove the working earth from the line, 
the mechanical clamp can be unscrewed and re- 
leased from the conductor by means of this rod. 
Even when an overhead line is earthed at each 
point of supply, it is necessary to place a working 
earth at each and eveiy position where work is 
being carried out on the line on account of the 
danger of the line becoming energized by induc- 
tion from other power lines and to safeguard 
against the charging of the line by atmospheric 
disturbances. Where the work entails breaking a 
conductor, for example, on the jumper at a sec- 
tioning point, it is necessary to provide a working 
earth on both sides of the working point. 

33.5 Saftey Earthing of Low Voltage Con- 
ductors — In some circumstances, it may be 
necessary to apply safety earthing to low voltage 
conductors in order to prevent danger. Such cir- 
cumstances may include, for example, work on 
capacitors or work on bare overhead crane trolley 



75 



IS : 3043 - 1987 



wires. Where the earthing of low voltage con- 
ductors is adopted, then the general principles set 
out in 33.2, 33.3 and 33.4 should be applied and 
due consideration should be taken of fault current 
levels (which can be as high or higher than on 
high voltage systems), when the size of earth 
conductor is chosen. 



34. MAINTENANCE 
TRODES 



OF EARTH ELEC- 



34.1 It is recommended that periodical check 
tests of all earth electrodes should be carried out. 
Records should be maintained of such checks. 

34.2 Where earth-leakage circuit-breakers are em- 
ployed, a check shall be kept on the associated 
earth-electrode by periodically operating the tes- 
ting device which is embodied in the earthed- 
leakage circuit-breaker. 

34.3 The neighbouring soil to the earth electrode 
shall be kept moist, where necessary, by periodic- 
ally pouring water through a pipe where fitted 
alongwith it or by pouring water in the immediate 
vicinity of the earth electrode. 

34.4 Substations and Generating Stations 

34.4.1 Records shall be kept of the initial 
resistance of substation and generating station 
earth electrodes and of subsequent tests carried 
out. 

34.4.2 Normally annual measurement of earth 
resistance of substation shall be carried out but 
local circumstances in the light of experience may 



justify increase or decrease in this interval but it 
should not be less than once in two years. 

34.4.3 Periodical visual inspection of all earth 
electrode connection, wherever available, shall be 
carried out to ensure their rigidity and other signs 
of deterioration. 

34.4.4 In rural substations, particularly those 
comiected to overhead high-voltage and low-vol- 
tage lines, greater reliance should be placed on the 
electrode system, and therefore facilities for testing 
the resistance of the electrode to general mass of 
earth, annually or as required by experience, 
should be provided. 

34.4.5 Where installations are earthed to a 
metal sheath of the supply cable, it shall be verified 
periodically that the earth-fault loop is in a satis- 
factory state. 

34.4.6 Where an installation is earthed to a 
cable sheath which is not continuous to the sub- 
station neutral (that is, there is an intervening 
section of overhead line without earth wire), a 
supplementary electrode system may be necessary. 
The adequacy of the electrode system shall be 
checked initially by an earth-fault loop test. 

34.4.7 The neighbouring soil to the earth elec- 
trode shall be kept moist, where necessary by 
periodically pouring water through a pipe where 
fitted along with it or by pouring water in the 
immediate vicinity of the earth electrode. 



SECTION 10 MEASUREMENTS AND CALCULATIONS 



35. CALCULATION OF EARTH 
CURRENTS 



FAULT suitable when the earth fault current is small 
compared to 3-phase fault current. 



35.0 General — The magnitude of the current 
that will flow in the event of a line-to-earth fauh 
on an earthed system is determined by the impe- 
dance from the source to the fauh plus the impe- 
dance of the earth return path, including the 
impedances of earthing transformers, resistors and 
reactors (.see IS : 5728-1970*). For interconnected 
systems, the calculation of the current may be 
complicated. 

35.1 Resistance Earthing 

35. LI When a single line-to-earth fauh occurs 
on a resistance grounded system, a voltage appears 
across the resistor nearly equal to the normal 
line-to-neutral voltage of the system. 

35.1.2 In low-resistance grounded systems, the 
resistor current is approximately equal to the 
current in the fault. Thus the current is practically 
equal to the line-to-neutral voltage divided by the 
resistance in ohms. This simple method is only 

*Guide for short-circuit calculations. 



35.2 In a resistance-earthed system with a single 
line-to- earth fault, the earth fault current may be 
computed from: 

/ 31 

where 

/g = earth fault current in A, 
X\ = system + ve sequence reactance in 
£l/phase including the subtransient 
reactance of the rotating machines, 

Xj = -ve sequence reactance as for X], 

Xo = zero sequence reactance as for X], 

Xn = reactance of neutral grounding 
reactor, 

Xgp = reactance of ground return circuits, 
and 

E = Hne-to-earth voltage in V. 

In most industrial and commercial systems 
without inplant generator X = Xi 



76 



18:3043-1987 



35.3 SoJid Earthing 

35.3.1 In this case, the fault current can be 
computed from: 

J ^ 3£ 

Aj + -^t + ^0 + 3 Xqp 

36. MEASUREMENT OF EARTH 
RESISTIVITY 

36.1 Resistivity of the Soil 

36.1.1 The resistivity of the earth varies within 
extremely wide limits, between 1 and 10 000 ohm 
metres. The resistivity of the soil at many station 
sites has been found to be non-uniform. Variation 
of the resistivity of the soil with depth is more 
predominant as compared to the variation with 
horizontal distances. Wide variation of resistivity 
with depth is due to stratification of earth layers. 
In some sites, the resistivity variation may be 
gradual, where stratification is not abrupt. Highly 
refined techniques for the determination of resis- 
tivity of homogeneous soil is available. To design 
the most economical and technically sound 
grounding system for large stations, it is necessary 
to obtain accurate data on the soil resistivity and 
on its variation at the station site. Resistivity 
measurements at the site will reveal whether the 
soil is homogeneous or non-uniform. In case the 
soil is found uniform, conventional methods are 
applicable for the computation of earth resistivity 
When the soil is found non-uniform, either a 
gradual variation or a two-layer model may be 
adopted for the computation of earth resistivity. 

36.1.2 The resistivity of earth varies over a 
wide range depending on its moisture content. It 
is, therefore, advisable to conduct earth resistivity 
tests during the dry season in order to get conser- 
vative results. 

36.2 Test Locations 

36.2.1 In the evaluation of earth resistivity for 
substations and generating stations, at least eight 
test directions shall be chosen from the centre of 
the station to cover the whole site. This number 
shall be increased for very large station sites of 
it, the test results obtained at various locations 
show a significant difference, indicating variations 
in soil formation, 

36.2.2 In case of transmission lines, the 
measurements shall be taken along the direction 
of the line throughout the length approximately 
once in every 4 kilometres. 

36.3 Principle of Tests 

36.3.1 Wenner's four electrode method is 
recommended for these types of field investiga- 
tions. In this method, four electrodes are driven 
into the earth along a straight line at equal inter- 
vals. A current / is passed through the two 



outer electrodes and the earth as shown in Fig. 33 
and the voltage difference V, observed between 
the two inner electrodes. The current / flowing 
into the earth produces an electric field propor- 
tional to its density and to the resistivity of the 
soil. The voltage K measured between the inner 
electrodes is, therefore, proportional to the field. 
Consequently, the resistivity will be proportional 
to the ratio of the voltage to current. The follow- 
ing equation holds for: 



^$K V 



9 "^ 



1 + 



2s 



2s 



V J« -h 4 ^8 V4 J» + 4 ^> 



.{I) 



where 

p = resistivity of soil in ohm-metre, 
5 - distance between two successive 
electrodes in metres, 

V ^ voltage difference between the two 
inner electrodes in volts, 

/ = current flowing through the two 
outer electrodes in amperes, and 

e == depth of burial of electrode in 
metres. 

36.3.1.1 If the depth of burial of the elec- 
trodes in the ground d is negligible compared to 
the spacing between the electrodes, then 



P 



2ns V 



.(2) 



36.3.1.2 Earth testers normally used for 
these tests comprise the current source and meter 
in a single instrument and directly read the resis- 
tance. The most frequently used earth tester is the 
four-terminal megger shown in Fig. 33. When 
using such a megger, the resistivity may be evalu- 
ated from the modified equation as given below: 



p - 2/7 



SR 



■(3) 



where 



p = resistivity of soil in ohm-metres, 

S = distance between successive electrodes 
in metres, and 

R = megger reading in ohms. 
36.4 Test Procedure 

36,4.1 At the selected test site, in the chosen 
direction, four electrodes are driven into the earth 
along a straight line at equal intervals, s. The 
depth of the electrodes in the ground shall be of 
the order of 10 to 15 cm. The megger is placed 
on a steady and approximately level base, the link 
between terminals PI and CI opened and the four 
electrodes connected to the instrument terminals 
as shown in Fig. 33. An appropriate range on the 



77 



IS : 3043 - 1987 



instrument is thus selected to obtain clear readings 
avoiding the two ends of the scale as far as possi- 
ble. The readings are taken while turning the 
crank at about 135 rev/min. Resistivity is calcula- 
ted by substituting the value of R thus obtained in 
the equation (3). In case where depth of burial is 
more than l/20th of spacing, equation (I) should 
be used instead of (3). 

36.4.2 Correction for Potential Electrode Resistance — 
In cases where the resistance of the potential 
electrodes (the two inner electrodes) is comparati- 
vely high, a correction of the test results would be 
necessary depending on its value. For this purpose, 
the instrument is connected to the electrodes as 
shown in Fig. 34. The readings are taken as 
before. The correction is then effected as follows. 



36.4.2,1 Let the readings of the megger be 
Rp with the connections as shown in Fig. 34 and 
the electrode spacing in metres. If the uncorrected 
value of soil resistivity is p' and the resistance of 
the voltage circuit of the instrument used to 
obtain R (as indicated inside the scale cover of 
the meter) is Rv, the corrected value of the earth 
resistivity would be: 

p = p' X (/?v + Rp)/Rv 



36.5 Testing of Soil Uniformity 

36.5.1 During the course of above tests, it 
would be desirable to get information about the 
horizontal and vertical variations in earth resisti- 
vity over the site under consideration for the 
correct computation of the resistivity to be used in 
the design calculations. The vertical variations 
may be detected by repeating the tests at a given 
location in a chosen direction with a number of 
different electrode spacings, increasing from 2 to 
250 metres or more, preferably in the steps 2, 5, 
10, 15, 25 and 50 metres or more. If the resistivity 
variations are within 20 to 30 percent, the soil in 
the vicinity of the test location may be considered 
uniform. Otherwise a curve of resistivity versus 
electrode spacing shall be plotted and this curve 
further analyzed to deduce stratification of soil 
into two or more layers of appropriate thickness 
or a soil of gradual resistivity variation. The hori- 
zontal variations are studied by taking measure- 
ments in various directions from the centre of the 
station. 

36.6 Computation of Earth Resistivity of 
Uniform Soil 

36.6.1 When the earth resistivity readings for 
different electrode spacings in a direction is within 
20 to 30 percent, the soil is considered to be 



CURftCMT 



PI 
■o 



CI 



P2 



C2 



MEGGEft 



POTENTIAL 
EUCTROOe 




CURRENT 

etecTRooE 



V/7J7777T77M777777777. 



-•»+««» S M 



FIG. 33 CONNECTIONS FOR A FOUR-TERMINAL MEGGER 




FIG. 34 TEST CONNECTION TO MEASURE THE SUM OF THE POTENTIAL ELECTRODE RESISTANCES 



78 



IS : 3043 - 1987 



uniform. When the spacing is increased gradually 
from low values, at a stage, it may be found that 
the resistivity readings is more or less constant 
irrespective of the increase in the electrode spac- 
ing. The resistivity for this spacing is noted and 
taken as the resistivity for that direction. In a 
similar manner, resistivities for at least eight 
equally spaced directions from the centre of the 
site are measured. These resistivities are plotted on 
a graph sheet in the appropriate directions choos- 
ing a scale. A closed curve is plotted on the graph 
sheets jointing all the resistivity points plotted to 
get the polar resistivity curve. The area inside the 
polar resistivity curve is measured and equivalent 
circle of the same area is found out. The radius 
0^ this equivalent circle is the average resistivity 
of the site under consideration. The average resis- 
tivity thus obtained may be used for the design of 
the earthing grid and other computations and the 
results will be reasonably accurate when the soil is 
homogeneous (see Fig. 35). 





2 




?^7?- 


is^_ 


7 Nl 


It 5^ 


^ A 


^^z^ 


is Zl 


rz _ 


5 y 




^'n 




_ ^^ 




S 



F!G. 35 POLAR CURVE 

37. MEASUREMENT OF EARTH 
ELECTRODE RESISTANCE 

37.1 Fall of Potential Method — In this method 
two auxiliary earth electrodes, besides the test 
electrode, are placed at suitable distances from the 
test electrode (see Fig. 3 6), A measured current 
is passed between the electrode A to be tested and 
an auxiliary current electrode C and the potential 
difference between the electrode A and the auxi- 
liary potential electrode B is measured. The resis- 
tance of the test electrode A is then given by; 



R^ 



I 



where 
R = 



of the test electrode in 



resistance 

ohms, 
V = reading of the voltmeter in volts, 

and 
/ = reading of the ammeter in amperes. 



37.1.1 If the test is made at power frequency, 
that is, 50 c/s, the resistance of the voltmeter 
should be high compared to that of the auxiliary 
potential electrode B and in no case should be 
less than 20 000 ohms. 



NOTE — In most cases, there will be stray currents 
Hewing in the soil and unless some steps are taken to 
eliminate their effect, they may produce serious errors 
in the measured value. If the testing current is of the 
same frequency as the stray current, this elimination 
becomes very difficult and it is better to use an earth 
tester incorporating a hand-driven generator. These 
earth testers usually generate direct current, and have 
rotary current-reverser and synchronous rectifier 
mounted on the generator shaft so that alternating 
current is supplied to the test circuit and the resulting 
potentials are rectified for measurement by a direct 
reading moving-coil ohm- meter. The presence of stray 
currents in the soil is indicated by a wandering of the 
instrument pointer, but an increase or decrease of 
generator handle speed will cause this to disappear. 



37.1.2 The source of current shall be isolated 
from the supply by a double wound transformer. 

37.1.3 At the time of test where possible, the 
test electrode shall be separated from the earthing 
system. 

37.1.4 The auxiliary electrodes usually consist 
of 12 -5 mm diameter mild steel rod driven up to 

1 m into the ground. 

37.1.5 All the test electrodes and the current 
electrodes shall be so placed that they are inde- 
pendent of the resistance area of each other. If 
the test electrode is in the form of rod, pipe or 
plate, the auxiliary current electrode C shall be 
placed at least 30 m away from it and the auxi- 
liary potential electrode B midway between them. 

37.2 Alternative Method 

37.2.1 The method described in 37.1 may not 
give satisfactory results if the test electrode is of 
very low impedance (one ohm or less). This 
applies particularly, while measuring the com- 
bined resistance of large installations. In these 
cases, the following method may be adopted. 

37.2.2 Two suitable directions, at least 90 deg- 
rees apart, are first selected. The potential lead is 
laid in one direction and an electrode is placed 
250 to 300 metres from the fence. The current 
lead is taken in the other direction and the 
current electrode located at the same distance as 
the potential electrode. A reading is taken under 
this condition. The current electrode is then 
moved out in 30-m steps until the same reading is 
obtained for three consecutive locations. The 
current electrode is then left in the last foregoing 
position and the potential electrode is moved out 
in 30-m steps until three consecutive readings are 
obtained without a change in value. The last 
reading then corresponds to the true value of 
earth resistance. 



79 



IS : 3043 - 1987 



.AMMETER 



CURR€NT 



TEST 
ELECTRODE 



POTENTiAL 
ELECTRODE 



» » 1m 




CURRENT 
ELECTRODE 



FIG. 36 METHOD OF MEASUREMENT OF EARTH ELECTRODE RESISTANCE 



38. MEASUREMENT OF 
IMPEDANCE 



EARTH LOOP 



38.1 The current, which will flow under earth 
fault conditions and will thus be available to 
operate the overload protection, depends upon 
the impedance of the earth return loop. This 
includes the line conductor, fault, earth-continuity 
conductor and earthing lead, earth electrodes at 
consumer's premises, and substations and any 
parallel metallic return to the transformer neutral 



as well as the trans fonner winding. To test the 
overall earthing for any installation, depending 
for protection on the operation of overcurrent 
devices, for example, fuses, it is necessary to 
measure the impedance of this loop under practi- 
cal fault conditions. After the supply has been 
connected, this shall be done by the use of an 
earth loop impedance tester. The neutral is used 
in place of the phase conductor for the purpose of 
the test. The open-circuit voltage of the loop 
tester should not exceed 32 volts. 



SECTION 11 DATA PROCESSING INSTALLATIONS 



39. EARTHING REQUIREMENTS FOR 
INSTALLATIONS OF DATA 
PROCESSING EQUIPMENT 

39.1 General 

39.1.1 Section 11 covers the special require- 
ments for the connection of data processing equip- 
ment to the electrical power installation of build- 
ings, where the data processing equipment has 
earth leakage current exceeding the limit specified 
in IS : 10422 - 1982* for equipment connected via 
a plug and socket. 

These requirements are intended to ensure the 
safety of personal in the presence of such leakage 
current. 

These rules apply to the installation up to the 
point of connection of the equipment as shown in 
Fig. 37. 

These rules do not consider installations for 
which the influence of lightning phenomena may 
exist. 

These rules do not consider the interconnection 
of equipment on different supply and earthing 
systems by data transmission lines. 

39.1.2 The requirements of this section may 
also be applied where installations, other than data 
processing such as those for industrial control and 
telecommunications equipment, carry high leakage 
current due to radio- frequency interference supp- 
ression filtering requirements. 



NOTE — Radio-frequency interference suppression 
filters fitted to data processing equipment may produce 
high earth leakage current. In such cases, failure of 
continuity in the protective earth connection may cause 
a dangerous touch voltage. The main purpose of this 
Code is to prevent this hazard. 

39.2 Definitions 

39.2.1 Data Processing Equipment — Electrically 
operated machine units that separately or assem- 
bled in systems, accumulate, process and store 
data. Acceptance and divulgence of data may or 
may not be electronic means. 

39.2.2 Low Noise Earth — An earth connection 
in which the level of conducted interference from 
external sources does not produce an unacceptable 
incidence of malfunction in the data processing or 
similar equipment to which it is connected. 

NOTE — The susceptibility in terms of amplitude/ 
.frequency characteristics varies depending on the type 
of equipment. 

'^9 23 High Leakage Current — Earth leakage 
current exceeding the limit specified in IS : 10422- 
1982* for equipment connected via a plug and 
socket. 

39.2,4 General Installation Requirements — The 
requirements of this clause apply where equipment 
having high leakage current is connected to any 
type of power system. The requirements apply to 
the installation as shown in Fig. 37. 



*Requirements and tests for safety of data processing 
equipment. 



* Requirements and tests for safety of data processing 
equipment. 



80 



IS : 3043 - 1987 



} 



iHSlAllAhONj 



eauiPMENT 

eQUfPMENf 



100P£0 
EQUIPMENT 

r 



{p— 



INDUS TRIAL i 

PIUC*50CKET t 
I 
I 
I 

mSTALlATlON I EQUIPMENT 




EQUIPMENT 



LOOPED 

EQUIPMENT 




*1 LOOPED 

if^sTALiATioNplHiJin^^^ tQ\)\pmm 




WfRtNG 
TERMINALS 



FIG. 37 EQUIPMENT-INSTALLATION BOUNDARIES 



Additional requirements are given for IT and 
TT systems in 39.2.4.4 and 39.3. 

NOTE i — On TNC systems, where the neutral and 
protective conductors are contained in a single conductor 
(PEN conductor) up to the equipment terminals, 
leakage current may be treated as load current. 

NOTE 2 ^ Equipment normally having high earth 
leakage current may not be compatible with installations 
incorporating residual current protective devices, as 
well as the standing residual current due to leakage 
current. The possibility of nuisance tripping due to capa- 
citor charging currents at switch-on shall be considered. 

Equipment shall be: 

a) stationary, and 

b) either pennanently connected to the 
building wiring installation or connected 
via industrial plugs and sockets. 

NOTE I — Industrial plugs and sockets are examples 
of suitable plugs and sockets. Plugs and sockets for 
general use are not suitable. 

NOTE 2 — It is particularly important for equip- 
ment with high leakage current that earth continuity 
should be checked at the time it is installed and after 
any modification to the installation. 

It is also recomiTiended that earth continuity 
be checked thereafter at regular intervals. 



Additionally, where leakage current ineasured 
in accordance with IS : 10422-1982* exceeds 10 niA, 
equipment shall be connected in accordance 
with one of the three alternative requirements 
detailed in 39.2.4.1 to 39.2.4,3. 

NOTE — Leakage current measurements prescribed 
by IS : 10422-1982* include likely undetected lault 
conditions within the equipment. 



39.2.4. 



High integrity earth connections 



NOTE — The aim of the requirements detailed 
below is to provide high integrity earth connections by 
using robust or duplicate conductors in association with 
permanent connections or robust connectors. 

Protective conductors shall comply with the 
following: 

a) Where independent protective conductors 
are, there shall be one conductor with a 
cross-sectional area of not less than 1 ^ 

mm" or two conductors with independent 
terminations, each having a cross-sectional 
area of not less than 4 mm^; 



* Requirements and tests for safety of data processing 
equipment. 



81 



IS : 3043 - 1987 



b) When incorporated in a multicore cable 
together with the supply conductors, the 
sum total cross-sectional area of all the 
conductors shall be not less than I ^ mm" 
and the protective conductors shall comply 
with Section 2; 

b) When incorporated in a multicore cable 
together with the supply conductors, the 
sum total cross-sectional area of all the 
conductors shall be not less than i f mm" 
and the protective conductors shall comply 
with Section 2; 

c) Where the protective conductor is installed 

in, and connected in parallel with a metal 
conduit having electrical continuity 
according to relevant Indian Standard 
specification on conduits for electrical 
purposes, a conductor of not less than 
2.5 mm' shall be used; and 

d) Rigid and llexible metallic conduits, 
metallic ducting and metallic screens, and 
armoiiiJ-ing which meet the requirements 
of Section 2. 

Each conductor specified in (a), (b), (c) and 
(d) shall meet the requirements of Section 2. 

39.2.4.2 Earth integrity monitoring — A pro- 
tective device shall be provided which will disco- 
nnect the equipment, in the event of a disconti- 
nuity occurring in the earth conductor, within 
the voltage/time limits prescribed by relevant 
standards. 

The protective conductors shall comply with 
Section 2. 

NOTE — The aim of the requirements detailed 
above is to monitor the continuity of the protective 
earth connection and provided means of automatic 
supply disconnection in case of failure. 

39.2.4.3 Use of double wound transformer — 
Equipment shall be connected to the supply via a 
double wound transformer ofother units in which 
the input and output circuits are separated, such 
as motor-alternator sets {see 4 0). 

The secondary circuit should preferably be 
connected as a TN system but an IT system may 
be used where required for the specific applica- 
tion. 

NOTE — The aim ofthe requirements above is to 
localize the path ofthe leakage current, and minimize 
the possibility of a break in continuity in this path, 

39.2.4.4 Additional requirements for TT 
system — The requirements below ensure that the 
leakage in normal operation of all equipment 
protected by one and the same protective device 
is less than half of that required to operate earth 
fault protective devices for the installation circuit. 

a) The total leakage current I\ (in amperes), 
the resistance of the earth electrode /?a 
(in ohms) and the. nominal operating 
residual current of the protective device 
/An (in amperes) shall be related as 
follows: 






U^ 



'^Ra 



b) If the requirements of (a) cannot be met, 
the requirements of 39.2.4.3 shall apply. 

39.3 Additional Requirements for IT 
Systems 

39.3.1 It is preferred that equipment with 
high leakage current is not connected directly to 
IT systems because of the difficulty of satisfying 
touch voltage requirements on a first fault. 

Where possible, the equipment is supplied by 
a TN system derived from the mains supply by 
means of a double wound transformer. 

Where it is possible, the equipment may be 
connected directly to the equipment may be 
connected directly to the IT system. This may be 
facilitated by connecting all protective earth 
connections for equipment using the IT system 
directly to the power system earth electrode. 

39.3.2 Before making direct connection to an 
IT system, installers shall ensure that equipment 
is suitable for connection to IT systems according 
to the declaration of the manufacutrer. 

39.4 Safety Requirement for Low Noise 
Earthing Connections 

NOTE — It may be found that the electrical noise 
levels on the protective earthing system of building 
installations cause an unacceptable incidence of mal- 
function on a data processing equipment connected to 
it. 

39.4.1 Whatever measures are taken to 
provide a low-noise earthing connection, it is 
required that exposed conductive parts of data 
processing shall be connected to the main earth- 
ing terminal. 

NOTE — The use ofseparate earth electrodes for 
simultaneously accessible exposed conductive parts is 
not permitted. 

This requirement shall also apply to metallic 

enclosures of Class II and Class III equipment, 

and to FELV circuits when these are earthed for 
functional reasons. 

Earth conductors, which serve functional pur- 
poses only, need not comply with Section 2. 

39.4.2 Other Special Methods — In extreme 
cases, if the safety requirements of 39.4.1 are 
fulfilled but electrical noise on the main earthing 
terminal ofthe installation cannot be reduced to 
an acceptable level, the installation has to be 
treated as a special case. 

The earthing arrangement has to provide the 
same level of protection as is generally provided 
by these requirements and particular attention 
should be given to ensure that the arrangement: 

a) provides adequate protection against over- 
current; 



82 



IS : 3043 - 1987 



b) prevents excessive touch voltages on the 
equipment and ensures equipotential be- 
tween the equipment and adjacent metal 
work or other electrical equipment, under 
normal and fault conditions; and 

c) meets the requirements relating to exces- 
sive earth leakage current, if appropriate, 
and does not invalidate them. 

40. EXAMPLE OF THE USE OF 
TRANSFORMERS 

40.1 Transformer incorporated in or Atta- 
ched to Unit — The transformer shall be 
connected in accordance with Fig. 38 in order to 



confine the earth leakage current in conductors 
within the unit, 

NOTE — No further special installation measures 
are necessary. 

40.2 Method of Connecting Transformers 
Physically Separate from. Units — The 

neutral point for the secondary circuit shall be 
connected to earth at the transformer and the 
earth connections between the equipment and 
the transformer shall comply with the require- 
ments of 39.2.4.1 or 39.2.4.2. 

Connections shall be as shown in Fig. 39. 




exposed conouctivi: 
Parts 



Single phase system depicted for ease. System may be 3-phase. 

Protection and control arrangements are not shown. 

C is the tllter capacitance. 

L\ and L2 or /V are connections to the incoming supply and P^ is the connection fix)m accessible parts of the 
equipment to the main earthing terminal of installation for both protective conductors of class 1 equipment and 
functional earthing conductors for class !1 equipment. 

FIG. 38 METHODS OF CONNECTING DOUBLE-WOUND TRANSFORMERS SITUATED 
WiTI-lIN OR ATTACHED TO SINGLE UNITS 



83 



IS : 3043 - 1987 



Lf 



12 

OR- 



P£- 



7K 



T 



c froA^i 



EXPOSED CONDUCTIVE 
PART 



1 



IZIC i lOAO I 



EXPOSED CONDUCTIVE 
PART 



Single-phase system depicted for ease. System may be 3-phase. 

Primary and secondary circuits must have means of control and protection. These are not shown. 

C is the filter capacitance. 

LI and L2 or /V are connections to the incoming supply and PE is the connection from accessible parts of the 
equipment to the main earthing terminal of the installation for both protective conductors of Class I equipment 
and functional earthing conductors of Class II equipment. 

FIG. 39 METHOD OF CONNECTING PHYSICAELY SEPARATED TRANSFORMERS 



84 



Bureau of Indian Standards 

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